CHEMISTRY COMMUNITY

Cecilia Cisneros 1F

I believe it would just be posted on chem community under chemical equilibrium, just because a lot of the application between the two is very similar.

Cecilia Cisneros 1F

I would solve this by using the equation nitrous acid forms hydrogen ions and nitrite. Then, you set up an ICE table. The initial value of nitrous acid is given, and the initial value of nitrite is .15 M because potassium nitrite completely dissociates. Initial for hydrogen ions is 0. Then, you can...

Cecilia Cisneros 1F

For phase changes, is it sufficient to know how to read the heating curve and know which are considered endothermic versus exothermic? Or what else is recommended?

Cecilia Cisneros 1F

The standard enthalpy of formation is just the standard enthalpy of a mole of a given substance. We are usually provided a table/chart in order to find the molecules corresponding standard enthalpy. Also quick note, make sure you pay attention to the state of the molecule! i.e (l), (aq), (s); it mak...

Cecilia Cisneros 1F

I think you multiply 0.25 by 200 to get the number of moles of the solution and then divide by 250 to get the concentration. Kinda unrelated question but where are you finding such a detailed answer key? The one I have only shows the answers I believe when you open Sapling you are able to download ...

Cecilia Cisneros 1F

If we are given the concentration of a solid would we include this in our equilibrium table? I know it does not affect the equilibrium constant. Does that mean we do not include it in the table? Thanks!

Cecilia Cisneros 1F

When we approximate K^-4 for cubic reactions we do not have to check that our solution is valid with the 5% rule right? That only pertains to acid/base equilibria?

Cecilia Cisneros 1F

In the textbook their are problems where we are solving for the pH of a polyprotic acid and it is advised to ignore the 2nd deprotanation of weak polyprotic acids because Ka2 is significantly less than Ka1. The only instance where we looked at the second deprotonation was for H2SO4 because it is a s...

Cecilia Cisneros 1F

To be safe I would approximate when the K is < 10^-4, as Dr. Lavelle had mentioned in lecture. Once you have solved for x, you would have to check the % ionized and make sure your percentage is less than 5% in order to confirm that your approximation is valid. However, If your percentage is greater ...

Cecilia Cisneros 1F

For a reverse reaction, you should take the inverse of the original K in order to find the equilibrium constant (AKA: new K value)

Cecilia Cisneros 1F

I tend to just use concentrations because it makes it much easier to be able to solve for equilibrium constants. I would say that is the safer bet (to use concentrations over moles) because there is a chance that you may forget to convert moles to concentration when finding equilibrium constants.

Cecilia Cisneros 1F

Essentially, yes. Q is measured at any point in the reaction that is not at equilibrium. However, K must be measured only at equilibrium.

Cecilia Cisneros 1F

5. “Acetic Acid (CH3COOH) is a common weak acid that gives vinegar its sour taste. At 25 degrees C the pKa=4.76. Suppose you titrate 100.mL of a 7.416 x 10 -3 M solution of acetic acid with an appropriate volume of 1.4682 x 10 -2 M NaOH to the equivalence point of 25 degrees C. What is the pH of the...

Cecilia Cisneros 1F

Does anyone know how to solve for percent ionization of an acid? I was using the formula:

% ionization = [H₃O+]_eq/[HA]₀ x 100%

but my answer was not matching the correct answer. Thanks for your help!

Cecilia Cisneros 1F

Vivian Hoang 1H wrote: $Ka=\frac{[H_{3}O^{+}][Al(H_{2}O)_{5}OH^{2+}]}{[Al(H_{2}O)_{6}^{3+}]} = 1.4 \cdot 10^{-5}=\frac{x^{2}}{0.055-x}\approx \frac{x^{2}}{0.055}$

This will give you value of X, which you can then plug in to find pH.

Where did you get the 1.4x10^-5?

Cecilia Cisneros 1F

This question asks to find the pH of 0.055M AlCl3. However, I am not sure where to find the Ka value in order to solve for x using the ice table. I did not see a Ka value for this in the table that I used for part 1. Does anyone know where to find it?

Cecilia Cisneros 1F

Is the answer key or the textbook incorrect because the question acts to solve for [OH-] given [H3O+], but for part c) there it says that the molar concentration is 3.1mol*L-1 and the solutions manual says that the concentration is 3.1x10^-3 how do we know which one we are supposed to use for this q...

Cecilia Cisneros 1F

Given the fact that Kw does not change and is always equal to (10^14) and that [H3O+]=[OH-] there is only one value that could equal to Kw which is (10^-7). Also, we know that pH of water is neutral = 7 so when you solve for [H3O+] we find that it is equal to (10^-7).

Cecilia Cisneros 1F

You would find the -2x, -x, and +2x from the balanced chemical equation you are using for that specific problem. The easiest way to know by what factor the compounds "change" is to look at the compound that has an initial concentration and subtract from there AKA (0.482 mol of N2, you know...

Cecilia Cisneros 1F

So just to make sure, we only omit x when the Kc is smaller than 1.0x10^-3 right? (of course only omit the x's being subtracted and I would assume the ones being added also, but not the ones being multiplied). Also, does Kc need to be significantly less than 1.0x10^-3 or just less than 1.0x10^-3? T...

Cecilia Cisneros 1F

So for part A, you begin by finding the Kc value of the equation we are looking at. From the equilibrium constant table, we see that Kc at 298 K for N2O4(g)----->2NO2(g) is equal to 6.1x10^23. We are looking at 2NO2(g)---->N2O4(g), so we want the reverse reaction, so the Kc of our reaction is 1/K o...

Cecilia Cisneros 1F

When we refer to the textbook table for the homework problems how do we know which equilibrium constant to use? I used a Kc value for a question asking to find the equilibrium concentrations, but the homework used the K value. I assumed that K by itself should only be used for finding partial pressu...

Cecilia Cisneros 1F

Q Scarborough 1b wrote:Hi there! I just realized that my sapling has a different value for the initial values of N2 and O2. So while mine would be k=36, it seems yours is K=4. Sorry for the confusion!

No worries! Thanks for your help too!

Cecilia Cisneros 1F

Yes, you are correct that K=4! To find the value at equilibrium, you need to write the K equation in terms of x. Draw an ICE table, and in the "change" row, you would subtract 2x in the NO column and add x in the N2 and O2 columns (because you are adding more product, the reaction shifts ...

Cecilia Cisneros 1F

Hi! Generally, changes in reactants are negative because we're using up the reactant to make a product. Therefore, changes in products are positive because we are making product by using up the reactant. I'm not really sure of any circumstances in which this rule would change, but please let me kno...

Cecilia Cisneros 1F

How do we know whether to be labeling the change as a positive or negative in the ice boxes? Does it depend on the initial concentration we are given/not given?

Cecilia Cisneros 1F

Hi there, For this problem you would set the initial value of NO as .900M. We can determine the value of K from the given concentrations, which turns out to be K=36. From there, you can use an equilibrium table to find the final NO, and use K=36, since that value does not change. Hope this helps. I...

Cecilia Cisneros 1F

Can anyone help me solve this problem I am having trouble figuring out where to begin. Would the initial value of NO be .900M and then how would we find the value at equilibrium is we are not given a change? Anything would be really helpful. Thanks!

Cecilia Cisneros 1F

I believe this refers to if the equilibrium is favoring products(right) vs. reactants (left). In terms of K that would depend on the ratio since K=[P]/[R] when K is large then equilibrium favors products and if K is small then equilibrium favors reactants.

Cecilia Cisneros 1F

I would pick a few UA sessions to attend weekly because this helps you to review the information for the week and helps you to understand what problems you may or may not understand. Also, I recommend doing all the practice problems from the outline because they are helpful to make sure you understa...

Cecilia Cisneros 1F

Could someone help explain the equilibrium constant in terms of stability? I got a bit confused when Dr. Lavelle mentioned this in lecture.

Cecilia Cisneros 1F

The ion with the larger atomic radius would be the one with the less positive charge. In this case, it would be Fe 2+. The reason being is that the one with the lesser positive charge has more electron repulsion between the electrons which result in the radius to be slightly expanded. For the more ...

Cecilia Cisneros 1F

How do we know between ions of the same element, which ion may have a larger atomic radius?

Fe2+ vs. Fe3+

Also, do isoelectronic elements have the same size radius? I know they have the same number of electrons, but I am not sure if they have the same atomic radius.

Cecilia Cisneros 1F

Would we be asked to see how an amphoteric compound will act as an acid or base in water? (pH <,>,= 7) OR do those types of questions just refer to analyzing salts?

Cecilia Cisneros 1F

I'm having a lot of trouble writing out the formulas for neutralization of acids and bases to salts does anyone have any tips!! Should we be writing them in the ionic forms like the textbook or can we leave them as compounds?

Cecilia Cisneros 1F

We are asked to write the neutralization of the salt by selecting an acid and base that would result in the formation of the given salt. How are we supposed to know that Zn(OH)₂ is written in this format? We are not given the charge for Zinc so I was just confused as to how we should know this?

Cecilia Cisneros 1F

Does the angular momentum quantum number "l" give only the number of angular nodes or does it give the number of nodal planes?

Cecilia Cisneros 1F

How do we know which atom the single electron would be placed when drawing the lewis structure of a radical?

Cecilia Cisneros 1F

Are there any H-bonds present in the compound: H₂SeO₄? Question 3F.1 says there are, but I didn't think there were any present.

Cecilia Cisneros 1F

If we are asked to determine the most stable compound should we be looking at the bond lengths or IMFs?

Cecilia Cisneros 1F

Is the hexadentate ligand "edta" charged? It is in the anionic section on the list of ligands on the class textbook, but I cannot seem to find the charge on it.

Cecilia Cisneros 1F

How do we know that Na+ and Cl- do not effect the pH. Dr. Lavelle mentioned that they don't, but I was unsure as to why that was the case.

Cecilia Cisneros 1F

If the coordination complex is an anion I understand that we add the suffix -ate to the transition metal. Do we still need to add the oxidation number in roman numerals at the very end of the coordination compound name?

Cecilia Cisneros 1F

What would be the formula to:
triamminediaquafluorocobalt(III) bromide?

I was practicing and came across this question, but I'm not sure if I'm right. Im thinking its:
[Co(NH₃)₃(OH₂)₂F] Br₂

Cecilia Cisneros 1F

How do we differentiate acids as being strong acids vs. weak acids? Dr. Lavelle listed some for us, but how do we determine this ourselves when given different compounds?

Cecilia Cisneros 1F

Is HCl or HBr the stronger acid? Dr. Lavelle said that HBr is the stronger acid, but the bond between HBr is longer than the bond between HCl. Longer bonds are weaker bonds. So now I'm not sure how HBr can be stronger than HCl.

Cecilia Cisneros 1F

When writing nitrito does it matter if we write it in the formula of the coordination compound as "ONO" or "NO2".
Would there be a difference as to which is more correct?

Cecilia Cisneros 1F

One of the UA's mentioned that a good note to have is that: the bis,tris,etc prefixes are mainly used for polydentate ligands. So if you have multiples of a polydentate ligand then you would use those prefixes.

Cecilia Cisneros 1F

Hemoglobin is primarily for oxygen transport in the blood and is the main mechanism used in our bodies for oxygen transport because it can hold more oxygen than myoglobin. On the other hand, myoglobin is only used when we are in need a quick bursts of oxygen for example in the tissue while we are wo...

Cecilia Cisneros 1F

When we are asked to give the formula of a coordination compound is there a specific order that the ligands are placed in the compound? I know that they must be written alphabetically in the compound name, but does it matter in which order we write the ligands in the formula?

Cecilia Cisneros 1F

Do lone pairs cause the bonding regions to be slightly smaller than the base geometry because they occupy a larger volume?

Cecilia Cisneros 1F

Hi! I attended Matthew Tran's workshop earlier, but I didn't get to stay at until the end. I was reviewing my answers with the answers from the workshop that someone kindly shared with me, but I was having trouble understanding the answer to 6a. "6. For each of the following molecules, identify...

Cecilia Cisneros 1F

Well the molecule has five areas of electron density. The VSEPR formula is AX2E3, so you know that it began with a base geometry of trigonal bipyramidal. Once you remove 3 bonds for lone pairs, you are left with a linear shape and the bond angle would be 180 degrees. I usually like to work my way do...

Cecilia Cisneros 1F

Can anyone explain what s-character is?

Cecilia Cisneros 1F

Lisa Wang 2J wrote:Please correct me if I am wrong, but I believe the size of the atom do not factor into the bond angle.

You could be totally right. I was just wondering if that had any effect on bond angle, but I could be completely overthinking it.

Cecilia Cisneros 1F

In the trigonal bipyramidal structure you should identify the 3 atoms that form the trigonal planar structure and these atoms are the in equatorial position and then the remaining two atoms (positioned above and below the trigonal planar) would be the two atoms in the axial positions. I think of it ...

Cecilia Cisneros 1F

In part c, the question asks: (c) What values are expected for the OSCl and ClSCl bond angles?

If the shape of the molecule is trigonal pyramidal are we able to conclude that all bond angles are less than 109.5 regardless of the size of the atom?

Cecilia Cisneros 1F

When two atoms are bound by one covalent bond this is known as a sigma bond. Whereas when two atoms are bound by two covalent bonds then there is one sigma bond and one pi bond.

Cecilia Cisneros 1F

Would the lowest energy structure be the one where the negative FC is on the most electronegative atom? For example the structure of NO2. It could be drawn two ways with a +1 charge on central N atom and a -1 on the O OR a +1 charge on the central N atom and a -1 charge on the other N atom. Which wo...

Cecilia Cisneros 1F

Also, I think you could tell which molecule is a radical based on the number of valence electrons between the atoms. If it is an odd number you know that there will be one electron that is unpaired, so it is a radical. At least that has worked for me!

Cecilia Cisneros 1F

I think C-Cl would experience dipole-dipole interactions. I believe that dipole-dipole interactions occur when there is a difference between electronegativities in a molecule. However, they do not occur in nonpolar molecules. At least that is the way I think of it. Yeah that's what I'm thinking too...

Cecilia Cisneros 1F

Would a bond between C-Cl be enough to experience dipole-dipole interactions? How do we determine whether there is a dipole moment present if we are just given the molecular formula and the molecule does not include N,O,F (the most electronegative atoms on the PT).

Cecilia Cisneros 1F

Can someone explain how the other two lewis structures are apart of the resonance structures? I thought resonance would have to be only with the lowest energy structure. In this case isn't the last one the only one that would be the lowest energy structure with all formal charges = 0. Therefore this...

Cecilia Cisneros 1F

When we are writing the electron configuration of (For Example: Br-) would it be acceptable to write it as either [Kr] or [Ar]3d^10 4s^2 4p^6? Is one preferred over the other?

Cecilia Cisneros 1F

London Dispersion Forces are synonymous with Induced Dipole-Induced Dipole, Van Der Waals, and Dispersion forces. These type of intermolecular forces tend to appear in all molecules, and when non polar molecules interact with each other they only experience LDFs.

Cecilia Cisneros 1F

I would say yes because NaCl would contain ion-ion forces and HCl would only contain induced dipole-induced dipole. The relationship between melting point and IMFs is that stronger IMFs have higher melting points.

Cecilia Cisneros 1F

Yes I agree with you! I think it may also be because the anxiety of the midterm tends to creep up on you as the midterm is approaching and you can't help but just worry about your grade! As long as you are studying the topics effectively I'm sure it will reflect in your exams! You got this :)

Cecilia Cisneros 1F

For this question I would think you can look at it both ways. The general guideline I follow for polarizability is that the bigger the anion the more polarizable. Since (I) is bigger then (F), (I) would be more polarizable than (F). In addition, bigger anions have more electrons than smaller anions ...

Cecilia Cisneros 1F

Do we only need to know that the charge and distance between charges is what effects the energy of an electron in a multi-electron system? Or would it just be more generally looking at effective nuclear charge for particular atoms?

Cecilia Cisneros 1F

If asked if a molecule has a dipole moment and the molecule has no net dipole moment since the dipoles cancel (for ex: CO2) would it still be considered to have a dipole moment?

Cecilia Cisneros 1F

Is there a rule that we should know to know that the ground-state electron confuguration of palladium is [Kr]4d^10?
Why is the 5s empty? Does this occur for every element with 8 electrons in the d-orbital?

Cecilia Cisneros 1F

Does an H-bond acceptor (O,N, or F) need to be attached to a hydrogen atom to participate in h-bonds?

Cecilia Cisneros 1F

I understand the relationship between IMFs and Boiling points. Where molecules that have stronger IMFs would be more tightly bound so you would need a higher boiling point to unbind electrons. However, what is the relationship between IMF strength and vapor pressure?

Cecilia Cisneros 1F

Hello,
I'm not sure if this true, but I had heard that everyone on the waitlist for Chem 14B would get into the class? With enrollment coming up I want to plan enrolling in my classes accordingly. Can anyone confirm this?

Cecilia Cisneros 1F

I believe another hint to let you know would be if an atom is bound to one of the high electronegative atoms (O,N,F). In the case where an atom is bound to one of those, then the electrons would be pulled closer to the more electronegative atom and cause a dipole moment on the compound. Dipole-Dipol...

Cecilia Cisneros 1F

Hey Cecilia, I would say for this problem you're going to need to use electronegativity. You're right in saying that you can use electronegativity to compare covalent bonds and polarizability to compare ionic bonds. It's just that in this case you're actually looking at a set of compounds with mixe...

Cecilia Cisneros 1F

Bigger anions will have a greater polarizability. Therefore, if you are ranking N3-, O2-, F-, and Cl- you could look at the atomic radius to deduce which anion is the largest and that would be the anion with the greatest polarizability. The order of increasing polarizability would be from smallest t...

Cecilia Cisneros 1F

I approached this problem looking at the polarizability of the compounds since these are ionic compounds. Which led me to believe that BBr3 would have the most primarily ionic bonds, since it is the smallest, most positively charged ion out of the three cations bound to Br (which means it has the gr...

Cecilia Cisneros 1F

If the value falls within 1.5-2 Dr. Lavelle mentioned in lecture that we would have to analyze the bond to see if it has a greater ionic or covalent character. However, for our purposes I believe they would give us a value that would be obvious in determining whether it is ionic or covalent. I.e. >2...

Cecilia Cisneros 1F

Hello, to my knowledge I don't believe there are any exceptions to London Dispersion Forces. All molecules interacting with each other should experience London dispersion forces!

Cecilia Cisneros 1F

I wanted to clarify the 4s is lower in energy that is why the orbital is filled with electrons first and that is why it comes before 3d when writing electron configurations. However, why is it that when an atom loses electrons is loses them from the 4s before the 3d? Could someone clarify?

Cecilia Cisneros 1F

Oh yes I think that was where I confused the expanded octet rule! Thank you!

Cecilia Cisneros 1F

In this question we are asked to draw the Lewis structure for BrF3. I drew my lewis structure with Br as the central atom and two of the F atoms had a single bond with Br and 6 lone pair electrons. For the third F, I drew a triple bond with Br and then 4 lone pair electrons. In addition, my Br had 2...

Cecilia Cisneros 1F

When asked for ground-state electron configurations would we write out the FULL electron configuration or the noble gas electron configuration? Would both be acceptable for example on an exam?

Cecilia Cisneros 1F

Looking at the row in the periodic table always helps me. When you count the rows [1,2,13,14,15,16,17,18] when you omit the d block. You can see that the number of electrons correspond to the row because there are 8 rows. Hope that makes sense! Quick question, can you explain why you omit the d-blo...

Cecilia Cisneros 1F

Can someone help explain effective nuclear charge and what it means. Also when asked about two different atoms how do we know which has a stronger/weaker effective nuclear charge. For ex) effective nuclear charge between Si and Al.

Cecilia Cisneros 1F

The L represents the number of lone pairs and it can be found by the number of electrons on an element that are not shared between another element.

Cecilia Cisneros 1F

Looking at the row in the periodic table always helps me. When you count the rows [1,2,13,14,15,16,17,18] when you omit the d block. You can see that the number of electrons correspond to the row because there are 8 rows. Hope that makes sense!

Cecilia Cisneros 1F

On sapling it directed to place the least electronegative atom as the center atom in a lewis structure. However, Dr. Lavelle mentioned in lecture that we should place the element with the lowest ionization in the center. Should we be following only one guideline and if so which one?

Cecilia Cisneros 1F

Wouldn't Boron also be included in the exception to the octet rule since it loses 3 electrons to become like He? Therefore, it would never have an octet?

Cecilia Cisneros 1F

Are the units to these answers supposed to be in L? Since the volume they give us is in liters, and when you use the equation
Minitial*Vinitial = Mfinal*Vfinal
the units you get for the final volume are liters, right?

Cecilia Cisneros 1F

In a practice problem I was trying to solve for the molecular formula but the ratio of the molar mass between the molecular formula and the empirical formula was not a whole number. What should I do in this instance? This was the problem: "After analyzing a sample of lactose there was found to ...

Cecilia Cisneros 1F

If you found the KE then you can use the equation:
KE = 1/2mv^2 to solve for the velocity of the electron.
Then you can use the de broglie equation:
wavelength = h/mv to solve for the wavelength of the electron.

Cecilia Cisneros 1F

You multiply the given uncertainty of your velocity when its a "±". Because the uncertainty is the difference between the ± 0.1m/s Uncertainty of velocity = difference = (0.2m/s -0.0m/s) = 0.2m/s Subtracting the difference is the same as multiplying by 2. You do not multiply 2 when they ex...

Cecilia Cisneros 1F

The theoretical yield would be the amount of product you find when converting from your limiting reactant based on a certain chemical reaction. It is the "perfect" reaction and amount of product that would come about from reacting your two reactants and there are no side reactions present....

Cecilia Cisneros 1F

I convert to m to be consistent with the velocity (m/s) and momentum (kg*m/s). I think that is always the best way to go! So long as all your units match up you should be good to go.

Cecilia Cisneros 1F

1B.15 The velocity of an electron that is emitted from a metallic surface by a photon is 3.6×10^3km⋅s−1. (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 2.50×10^16Hz. How much energy is requir...

Cecilia Cisneros 1F

Can someone help me understand part C for this question? In the manual it says its based on the wave model, but if someone could explain it further I would appreciate it!

Cecilia Cisneros 1F

As a general rule of thumb should we be doing our calculations by using the entire number that we get on our calculator? I was solving a photoelectric effect problem and my answer was not matching with one of the workshop worksheet answers. They rounded to 2 sig figs when they plugged values other t...

Cecilia Cisneros 1F

Is there a big difference if we use nf and ni instead of n1 and n2 in the Rydberg equation? Thanks in advance!

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