CHEMISTRY COMMUNITY

Arezo Ahmadi 3J

[H3O+] was not included in the equation because when Ba(OH)2 dissociates, it only dissociates into Ba2+ and OH-. Ba2+ is a spectator ion, so it does not affect our calculations. Then, you can use stoichiometry to see that for 1 mole of Ba(OH)2, there are 2 moles of OH-. Thus, we multiply the concent...

Arezo Ahmadi 3J

When water is neutral, that means the concentration of H3O+ and OH- are equal. Since Kw has to be constant, [H3O+] and [OH-] have to be the same value to equal Kw, which is 1.0 x 10^-14. Thus, [H3O+] = 1.0 x 10^-7 and [OH-] = 1.0 x 10^-7.

Arezo Ahmadi 3J

Since Ba(OH)2 is a strong base, Ba2+ is a spectator ion. Spectator ions end up being cancelled out, so that is why it is not included in the Kw equation.

Arezo Ahmadi 3J

For the concept of Le Chatelier's Principle, a reaction that is not at equilibrium is going to want to achieve an equilibrium state, which means that the reactants are going to be used up and the products are going to form. Since reactants are being used up, the change in their concentration is decr...

Arezo Ahmadi 3J

Hi, if the system is compressed, you are increasing pressure and decreasing volume, so you are right! After establishing this, you can look at the number of moles of gas on each side of the equation. Aqueous solutions would not be included in this observation.

Arezo Ahmadi 3J

Usually in the equation, the phases are stated next to the reactants and products. Solids are identified as (s), liquids as (l), aqueous solutions as (aq), and gases as (g).

Arezo Ahmadi 3J

K is the value for the equilibrium constant, which means that it is a value that is specific to when a reaction is at equilibrium. However, Q is the reaction quotient to represent the ratio of products to reactants when the reaction is not at equilibrium. You can compare the value of Q to the value ...

Arezo Ahmadi 3J

Every chemical reaction should have the potential to reach equilibrium given that it has the right conditions to reach the reaction's equilibrium constant. This could depend on things like time, since reactions need time to occur and reach that equilibrium constant.

Arezo Ahmadi 3J

Reactants would have a negative sign and products would have a positive sign for x since reactants are being used up and their change is decreasing, while products are being produced and their change is increasing.

Arezo Ahmadi 3J

If you increase the concentration of products at equilibrium, then that means the equilibrium constant needs to remain constant. Since the number in the numerator is now increased, by Le Chatelier's Principle, we need to increase the number in the denominator too, which represents the concentration ...

Arezo Ahmadi 3J

Since 60% of the hydrogen gas reacted, this means that there is 40% left at equilibrium, so you would have 40% of the initial concentration of hydrogen gas left at equilibrium (which is stated in the problem as 0.400 mol/3.00 L) and thus calculate it from there using the equilibrium table.

Arezo Ahmadi 3J

I know that specifically when the equilibrium constant is 0, we would not see that because of how that would mean the product concentration would have to be 0 at equilibrium, which is not reasonable.

Arezo Ahmadi 3J

When given a reaction with aqueous reactants or products, do we need to first write out the net ionic equation in order to write out the K expression? Or do we just leave the aqueous reactants and products in their original form? Thank you in advance!

Arezo Ahmadi 3J

I was wondering, if a reaction is not at equilbirium, does that mean the concentrations of a reactant and a product would be different between the forward and reverse reactions? I know in lecture today Dr. Lavelle mentioned that it doesn't matter if you are looking at the forward reaction of a syste...

Arezo Ahmadi 3J

You would just not include the solid as is the standard procedure for writing out the equation. Both solids and liquids are not included in the equilibrium constant. You would, however, include the gas and aqueous solution.

Arezo Ahmadi 3J

Anytime that the given heat is negative, you would identify this as an exothermic reaction because heat is being given off. Thus, if heat is being given off, or produced, this means that heat is a product. An increase in the products means that to return to equilibrium, a shift to the left, or a shi...

Arezo Ahmadi 3J

For number 19 in the Chemical Equilibrium Part 3 Post-Assessment, the question asks: 0.482 mol N2 and 0.933 mol O2 are placed in a 10.0 L reaction vessel and form N2O (dinitrogen oxide): 2 N2(g) + O2(g) ⇌ 2 N2O(g) KC = 2.0 x 10^-37 What is the composition of the equilibrium mixture? I understand how...

Arezo Ahmadi 3J

I think that the use of Kc and Kp is there to clarify if you are dealing with concentration or pressure, respectively, whereas K is just a broad term for the reaction.

Arezo Ahmadi 3J

Oh that makes sense! Thank you! Just a quick question, though, so why does Q not change even though more products are being produced? I was just confused by how the reactant/product concentrations stayed the same since we're looking at the reaction immediately after an increase in temperature and m...

Arezo Ahmadi 3J

Hi, Note, it is important to remember the equilibrium constant K is dependent only on temperature. Changes to initial conditions such as initial concentrations, initial partial pressures, or introductions of catalysts do not affect the value of K . When the temperature is increased for the system, ...

Arezo Ahmadi 3J

The only time that K is able to change is with a change in temperature. The problem asks about what happens immediately after an increase in temperature. This means that the reactants/products have not had a chance to respond yet, and their concentrations remain the same as before the increase in t...

Arezo Ahmadi 3J

Hi, I was confused about the reaction quotient component of number 7 from Sapling. The question gives an endothermic reaction and asks "What can be said about Q and K immediately after an increase in temperature?" I understood the second part of the question in which the system would shift...

Arezo Ahmadi 3J

If you draw out the Lewis structure for CO2 and find the hybrid orbitals, I get that the sigma bond for the bond between C and O is (C 2sp, O 2sp^2). But for the pi bond, would it be (C 2p^2, O 2p)? I want to make sure that this carbon part of the pi bond is correct because if carbon is sp hybridize...

Arezo Ahmadi 3J

One of the methods that helps me initially determine if an acid is strong or weak is by referring to the seven strong acids that can be found in the textbook. Since HCN is not one of them, that helps me identify it as a weak acid.

Arezo Ahmadi 3J

Electronegativity is important when considering acidity because the more electronegative an element is, the more electron pulling power it has and thus it can distort a negative formal charge from atoms such as an oxygen in a molecule. Thus, this pull from a highly electronegative element is able to...

Arezo Ahmadi 3J

Yes, both of them can be considered amphoteric because they each have a H+ and a negative charge. This means that they can act as an acid by donating an H+ or act as a base by accepting an H+ and becoming a neutral compound.

Arezo Ahmadi 3J

Just to make sure, are cations from Group 1 and 2 acids? I know that they do not affect pH, and more so for Group 1, but it was mentioned today that the cations Fe3+, Cr3+, Al3+, Cu2+, and Ni2+ act as Lewis acids. What is the importance of this statement?

Arezo Ahmadi 3J

It was discussed that when the solution is more basic as the pH is greater than the pKa, then the weak acid gives off a proton. Is the basic solution itself the weak acid that is being referred to? I thought that bases accept protons, so I'm slightly confused on this.

Arezo Ahmadi 3J

I'm still a bit confused on the pH and pKa example about the biological acid, HA, from today's lecture. Are pH and pKa the same concept describing one acid? Or are there two different acids in this example? Could you clarify the molecules involved and why we have these two different values? Thank you!

Arezo Ahmadi 3J

You can find the dentate number by looking to see how many lone pairs are available in a ligand to bind with a transition metal. For example, if there is only one lone pair that can be donated, then it is a monodentate. Additionally, if there were 2 lone pairs available to be donated, then the liga...

Arezo Ahmadi 3J

In today's lecture, Dr. Lavelle mentioned the -COOH groups in the biological molecules. I was just wondering, when he says that these compounds need to be in the presence of water, does that mean that the entire compounds are not acids until they are placed in water and thus the hydrogen is donated ...

Arezo Ahmadi 3J

I was doing a UA worksheet when I saw that we had to write the formula for this coordination compound: Diamminedichloronickel. It is supposed to be [NiCl2(NH3)2], but does anyone know why the Cl2 is before the (NH3)2? I was going in the order of the name, but why are these two switched inside the br...

Arezo Ahmadi 3J

In today's lecture, Dr. Lavelle mentioned both partial pressure and pH as having effects on oxygen. I understand that if pH is changed, this could have an effect on shape and thus the binding of oxygen, but what is the effect of changing partial pressure? Could you explain why oxygen transport and t...

Arezo Ahmadi 3J

Could someone explain the importance of the porphyrin ligand? I'm not sure what its components are in relation to the myoglobin structure. Is it describing all four of the nitrogen molecules, or something else? Thank you!

Arezo Ahmadi 3J

I was wondering if the order of transition metals in the periodic table has anything to do with their characteristics for biological functions. Dr. Lavelle mentioned today that chromium was one of our earlier exceptions for electron configurations when he was mentioning its biological function, and ...

Arezo Ahmadi 3J

The coordination number is simply the number of bonds that are attached to the transition metal. In an example used in lecture today, [Ni(CN4)]2- has a coordination number of 4.

Arezo Ahmadi 3J

When discussing the myoglobin structure in today's lecture, it was mentioned that it is a tetradentate structure. I was confused on this description because I thought that there were 6 bonds because iron likes to be in an octahedral structure, but is this not correct? Could you expand on why myoglob...

Arezo Ahmadi 3J

Hi, I was wondering if, since cisplatin has two chlorine atoms that bind to the guanine of G-C base pairs, cisplatin is a bidentate ligand? Or is the nitrogen in the guanine the ligand in this coordination compound? Could someone clarify what is happening between the guanine and cisplatin molecules ...

Arezo Ahmadi 3J

Hi, I was wondering if anyone had any tips for determining the bond angles for the trigonal bipyramidal and octahedral structures when lone pairs are added? It is a bit hard for me to visualize how different numbers of lone pairs affect those shapes in terms of the bond angles. If anyone has any adv...

Arezo Ahmadi 3J

Hi, I was wondering how in lecture today (11/25/2020), it was able to be determined that a transition metal like cobalt had a charge of 2+. Could you explain where that oxidation number came from when looking at the bonding inside the coordination sphere? Thank you in advance!

Arezo Ahmadi 3J

In this example, there are four electron densities with two being lone pairs, and this means that there is going to be repulsion from these lone pairs, making the bond angles less than 109.5 degrees. As a result of this shape, the name of the shape is bent instead of linear. Hope this helps!

Arezo Ahmadi 3J

In addition to what has been said, cisplatin and transplatin are geometric isomers, which means that the positioning of the atoms differ between them. Since cisplatin and transplatin only have sigma bonds, they are free to rotate and are not as locked into their individual structures, which would me...

Arezo Ahmadi 3J

One characteristic that makes a coordination compound stronger than another is if there is a chelating ligand involved, as there would be more than one location of bonding in the coordination compound. Thus, if there are at least two lone pairs in the ligand that can bond, this would make the coordi...

Arezo Ahmadi 3J

You can find the dentate number by looking to see how many lone pairs are available in a ligand to bind with a transition metal. For example, if there is only one lone pair that can be donated, then it is a monodentate. Additionally, if there were 2 lone pairs available to be donated, then the ligan...

Arezo Ahmadi 3J

It would be determined by the formula that is given to you in the square brackets. Not all of the transition metals will have 6 bonds depending on the ligand, since there are also tetrahedral and square planar structures that are common as well, which have four bonds.

Arezo Ahmadi 3J

At a UA session, it was said that sigma bonds are stronger than pi bonds, but I was wondering why exactly this is. I thought that since pi bonds form as a result of double bonds, and sigma bonds can be present in single bonds, that pi bonds would be stronger, but this is not the case. Could you expl...

Arezo Ahmadi 3J

At the end of Wednesday's lecture this week, the molecule cis-dichloroethene was shown to be a polar molecule because its dipole moments did not cancel out, but I was wondering why the double bond plays a role in this. I thought that the dipole moments would cancel out since the molecule appears to ...

Arezo Ahmadi 3J

Hi, for me, it helps to look at charts that are organized online when I am memorizing the VSEPR shapes. From there, I am able to keep practicing and testing myself on knowing the names for linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral, and then when it comes to the names...

Arezo Ahmadi 3J

Hi, I'm confused about the difference between molecular shape and geometry, are they the same concept or do they concern the number of lone pairs on a central atom? I know that in lecture Dr. Lavelle said that lone pairs affect shape but not the name, so I was hoping someone could explain why. Thank...

Arezo Ahmadi 3J

When determining the molecular shape of a molecule, do you have to consider the most stable structure for its Lewis structure?

Arezo Ahmadi 3J

In the lecture from Monday, Dr. Lavelle said that for SO3 ^2-, its lone pair makes the bond angle less than 109.5 degrees. Does this mean that the bond angle in the region where the lone pair is would be bigger than 109.5 degrees and the other bond angles below are the ones that are less than 109.5 ...

Arezo Ahmadi 3J

If you are drawing the Lewis structure of a neutral molecule, there would not be any brackets because there is no charge associated with that molecule, but if you are drawing the Lewis structure of a charged molecule, such as NO3 -, then there would be brackets needed to be drawn so that you can ind...

Arezo Ahmadi 3J

Dissociation energy correlates to the strength of the bonds because it is how much energy is needed to break a bond, and so the more unpaired electrons you have, the weaker the bond is because those lone pairs are repelling each other. This, in turn, decreases the dissociation energy of the bond. Ho...

Arezo Ahmadi 3J

No, for now it does not matter where the hydrogen atom goes as long as you place the nitrogen atom as the center atom because it has the least electronegativity when compared to fluorine. Other than that, the fluorine atoms and hydrogen atoms can be placed around the nitrogen atom. Hope this helps!

Arezo Ahmadi 3J

Yes, and we can look at this from seeing that boron can form 3 bonds, since it has 3 valence electrons and thus can form these 3 bonds with six electrons. As a result, this means that it is an exception to the octet rule and so it is electron deficient.

Arezo Ahmadi 3J

How do we identify if a neutral molecule is a Lewis Acid or Lewis Base? I am familiar with identifying Lewis Acids and Lewis Bases when there is a charge on a molecule like for polyatomic ions, but how do you know if a molecule is a proton acceptor or proton donor when there is no charge?

Arezo Ahmadi 3J

You are correct in your reasoning that the atom with the lowest ionization energy should be in the center of a Lewis structure. Oxygen actually has a higher ionization energy than nitrogen, and this is why nitrogen goes in the center of the Lewis structure. You can identify the ionization energy tre...

Arezo Ahmadi 3J

You can identify the number of valence electrons that carbon has by looking at its electron configuration. The number of electrons that it has in its outermost shell is 4, which is indicated by the electron configuration [He]2s^2 2p^2. Hope this helps!

Arezo Ahmadi 3J

For formal charges, a type of problem might be asking for the most dominant or likely structure for a Lewis structure, and in this case, the problem is looking for the Lewis structure with atoms that have the smallest formal charges. Hope this helps!

Arezo Ahmadi 3J

The atom that has the least electronegativity would be the central atom. In this case, nitrogen has the least electronegativity if you follow the trend of electronegativity increasing across a period and up a group in the periodic table, so nitrogen is the central atom for this Lewis structure.

Arezo Ahmadi 3J

Hi, I was confused on part (c) of this question from the textbook: Draw the Lewis structure of (a) CCl4 ; (b) COCl2 ; (c) ONF; (d) NF3 .

For part (c), why is it that the double bond goes between N and O instead of N and F? How do you know where to place the double bond?

Thank you in advance!

Arezo Ahmadi 3J

Hi, I was reviewing a problem and I was wondering why a compound such as AgF has a lower covalent character than BeCl2, because in the compound BeCl2, these elements are much more farther apart in the periodic table than the distance between the elements in AgF. Could I have the reasoning behind wha...

Arezo Ahmadi 3J

Hi, I was looking at this problem and had a question about one of the statements: Which of the following statements are true for many-electron atoms? If false, explain why. (a) The effective nuclear charge Zeffe is independent of the number of electrons present in an atom. (b) Electrons in an s-orbi...

Arezo Ahmadi 3J

I was wondering if the energy between subshells, such as going from the 2s-orbital to the 2p-orbital, increases, decreases, or stays the same. I know that the energy between shells such as from the 2p-orbital to the 3s-orbital, but what happens when you stay within the same shell? I am wondering abo...

Arezo Ahmadi 3J

Nitrate is an anion which means that it does indeed have an extra electron, and you have to account for this by adding it to the number of electrons. An easier way to remember this is that whenever you just see a charge on an ion, that would be the number of electrons you either add or do not add to...

Arezo Ahmadi 3J

When I was watching today's lecture, Dr. Lavelle went over observed bond lengths and specifically for the nitrate ion, but I was wondering how this relates to the bond length values he gave for the double bond versus the single bond in a nitrate ion as well as if this is related to bond order. In re...

Arezo Ahmadi 3J

The effective nuclear charge plays a really important role in whether or not it can better effectively shield electrons from the nucleus. In the 3 shell, all the elements are located in the same shell, and that might be where the confusion is regarding the size of the shells. Instead, as you noted, ...

Arezo Ahmadi 3J

Hi, you can look at it in terms of the shells. The 4th shell should have a higher energy than the 3rd shell, and this energy increases as the shell number increases. This is why when we write electron configurations, we need to write 3d before 4s, because electron configurations need to be written s...

Arezo Ahmadi 3J

Hi, I was confused on this question: Consider a single photon with a wavelength of \lambda , a frequency of \nu , and an energy of E. What is the wavelength, frequency, and energy of a pulse of light containing 100 of these photons? I was confused on the part of this question in which 100 is not sup...

Arezo Ahmadi 3J

Hi, so you could definitely see a problem where a +/- is given with a percentage. In this case, you would have to double the percentage value as you have been doubling uncertainities after the +/-. You just need to then multiply this doubled value of the percentage to the value, which in your case, ...

Arezo Ahmadi 3J

When you are using 2.014 amu, this essentially means grams per mole, so you would have to convert to grams by dividing by Avogadro's Number. Then, always make sure you check the context of the question because when using equations such as DeBroglie Equation, the mass needs to be in kilograms, so div...

Arezo Ahmadi 3J

E=hv is an equation that is measured in Joules per photon, as this equation can be used to find the energy of the incident light, which contains photons. Therefore, when considering these units of Joules per photon, the energy of ejected electrons cannot be found using the equation E=hv, particularl...

Arezo Ahmadi 3J

In your work in which you calculated for the uncertainty in velocity, you are dividing by 10m again when you should be dividing by the mass of the car, which is 2150 kg. This would then look like: delta p: h/4pi(10 m) = 5.28 x 10^-2 kgm/s and delta v would look like: delta v: (5.28 x 10^-2 kgm/s) / ...

Arezo Ahmadi 3J

Hi, for your steps in the second part of the question, you need to take the value that you calculated for electron affinity, and because it is in eV/atom, you need to convert using the conversion 1 eV = 1.602 x 10^-22 kJ, and then from here, you have to multiply by Avogadro's number in order to conv...

Arezo Ahmadi 3J

Hi, I was confused on the value for the Rydberg Constant because I have been using 3.29 x 10^15 s^-1, which I then used in the equation v = R {1/(n1)^2-(1/(n2)^2}, with n1 being the initial n and n2 being the final n, but I also saw a value for the Rydberg Constant be 1.0974 x 10^7 m^-1 in the equat...

Arezo Ahmadi 3J

In addition to the steps given about calculating for the Joules of energy, an important concept to remember that you can apply when calculating for other problems in the future is to always look at the units. When you calculate for energy after finding the wavelength in meters and plugging in your v...

Arezo Ahmadi 3J

Hi! An alternative way that I ended up solving this problem was I used the equation Change in Energy = Efinal - Einitial. We can calculate Efinal using Ef= -hR/(n final)^2. Since we know that n has to equal 1 because the wavelength is in the UV region which is the Lyman series and thus correlates to...

Arezo Ahmadi 3J

In addition to the given information about the smallest wavelength that would still be measurable is 10^-15 m, Dr. Lavelle said that you wouldn't have to worry about it being too close or potentially being ambiguous, the wavelength you calculate will definitely be clear in order to make sure you und...

Arezo Ahmadi 3J

Hi, while watching the end of today's lecture (10/19/2020), I understood how we were deriving DeBroglie's equation by setting E=hv to E=pc, but I was confused about the concept of whether or not DeBroglie's Equation can still be applied to photons. I know that there was some debate before the DeBrog...

Arezo Ahmadi 3J

Hi, I'm confused on this question: The velocity of an electron that is emitted from a metallic surface by a photon is 3.6 x 10^3 km/s. (a) What is the wavelength of the ejected electron? (b) No electrons are emitted from the surface of the metal until the frequency of the radiation reaches 2.50 x 10...

Arezo Ahmadi 3J

Hi, I was confused about the wave model versus the model that we are usually shown in the photoelectric experiment. I'm having trouble understanding why increasing the intensity of the light doesn't affect the energy of the photons and how we know that this model isn't a wave model. What is the diff...

Arezo Ahmadi 3J

The numbers in the names 2-butanone and 3-bromopropane are not related to the stoichiometric coefficients of each molecule. Instead, they just refer to the structures of the skeletal structures to let you know a quantity of a certain group or element in the skeletal structure's composition. For this...

Arezo Ahmadi 3J

Hello, I am confused on how to approach this question: 21. The electron is not confined to the nucleus and we now know that the size of an atom is determined by its electrons outside of the nucleus. For hydrogen its measured atomic diameter is 145,000 times its nuclear diameter of 1.7 x 10^-15 m. In...

Arezo Ahmadi 3J

Hi, I was wondering about the concepts behind this question: 15. When waves interact in-phase is that constructive or destructive interference? A. Destructive interference B. Constructive interference I understood waves interacting in-phase to be destructive interference because the waves are overla...

Arezo Ahmadi 3J

Hi, I'm having trouble understanding the concept behind this question: 14. For large everyday objects does Heisenberg's uncertainty (indeterminacy) principle play any measurable role? A. Yes, the uncertainties in position, speed, and momentum of a stationary object are noticeable or measurable. B. Y...

Arezo Ahmadi 3J

For this problem, the number of moles of 3-methyl-3-hexanol are equal to the number of moles of 2-butane because based on the information that we are given, the equation is balanced and we can assumed that the ratio is one mole of 3-methyl-3-hexanol to one mole of 2-butane. Additionally, we know tha...

Arezo Ahmadi 3J

Hi, I was confused about this question: In medicine it is sometimes necessary to prepare solutions with a specific concentration of a given ion. A lab technician has made up 100.0 mL of a solution containing 0.50 g of NaCl and 0.30 g of KCl, as well as glucose and other sugars. What is the concentra...

Arezo Ahmadi 3J

Hi, so for this problem, you will need to find the molar mass of Sn, and use this to convert 1.50 g of Sn into moles of Sn. Then, because they give you the value 28.35g which represents the crucible and product, you'll need to subtract the mass of the crucible, or 26.45g, from 28.35g in order to fin...

Arezo Ahmadi 3J

Hi, I need help with this question: Solve this exercise without using a calculator. The reaction 6 ClO2(g) + 2 BrF3(l) -> 6 ClO2F(s) + Br2(l) is carried out with 12 mol ClO 2 and 5 mol BrF 3. (a) Identify the excess reactant. (b) Estimate how many moles of each product will be produced and how many ...

Arezo Ahmadi 3J

Adding on to the previously great explanations given, when I approached this problem, I realized that the skeletal structures that were given also needed to be fully completed so that you could calculate the molar mass. To find the molar mass of each of the structures, take a look at the corners and...

Arezo Ahmadi 3J

In order to find only parts of the concentration, you'll need to look at each molecule that has that ion that you're looking for. So for part a, you'll notice that KCl, K2S, and K3PO4 all have the K+ ion, so we're going to have to use the masses we were given for each of them. Now, you are going to ...

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