CHEMISTRY COMMUNITY

Aaina 2D

Hi! Just to confirm, do strong acids/bases not have equilibrium constants because they dissociate completely?

Aaina 2D

Hi! So, the first part asks you to solve for the pH of the desired solution, which is 200.0 mL of a 0.025 M HCl solution. HCl is a strong acid, so it completely dissociates into H+ and Cl-, and so you can calculate the pH by taking the negative log of [H+] or [0.025]. For part b, to find the actual ...

Aaina 2D

MHarrold_1E wrote:[ionized amount]/[initial amount] x100 = percent ionized.

But would you divide the [H3O+] ionized or the [CB] ionized by the initial amount?

Aaina 2D

Hi! I was just thinking, and I'm not really sure what an equilibrium constant actually means. What exactly does it tell us?

Aaina 2D

Anna Yang 1A wrote:On the syllabus it says that the midterm is on a Friday during class time. Is this true because for 14A last quarter, it was during our respective discussion sections?

Hi! Yes this is true. Midterms this quarter will be held during our class lecture time on Friday.

Aaina 2D

To add on, for strong acids/bases, they ionize or dissociate completely, and therefore their percent ionization is 100%. For weak acids and bases (which don't ionize completely), you take the equilibrium [OH-] (for bases) or the [H3O+] (for acids) and divide it by the initial concentration of the we...

Aaina 2D

To add on to the above response, percent ionization is just how much of the acid dissociates into H3O+.

Aaina 2D

Hi! I would definitely attend some UA sessions if you can. I find the worksheets they give really helpful. I would also suggest doing textbook problems as we go through each unit, as it gives you good practice.

Aaina 2D

According to Le Chatelier's principle, chemical reactions adjust so as to minimize the effect of changes. Therefore, I believe decreasing the temperature will increase the K value (more products) for an exothermic reaction and decrease the K value for an endothermic reaction. Please correct me if I...

Aaina 2D

Hi! I would just go with anything smaller than 10^-3, because that is what the textbook goes by too. Dr. Lavelle also mentioned for anything smaller than 10^-4, the value of x is DEFINITELY negligible. It's just more certain, but I would still stick to anything smaller than 10^-3.

Aaina 2D

Hi! In the equilibrium constant of water, why are the concentrations of H3O+ and OH- the same (1x10^-7 for both)? I didn't really understand the reasoning.

Aaina 2D

Hi! I was sent to a page where it asked me to pay again or to "use credit". The "use credit" is the option you should choose, but this option will only be there if you purchased a full year access to sapling or more than one term.

Aaina 2D

Using ICE as a method is a bit more involved than what explanation allows, but a brief overview is what follows. ICE is an acronym standing for Initial, Change, and Equilibrium. When given a chemical equation and their initial concentrations, you can input their initial concentrations into Initial ...

Aaina 2D

Hi! As far as I know, temperature is one of the only things that affect the equilibrium constant (K). The way I look at how temperature affects a reaction is by whether it's exothermic or endothermic. If the reaction is endothermic, heat would technically be on the reactants side, and if you increas...

Aaina 2D

Hi! Is the equilibrium constant for a certain reaction the same every time regardless of the initial concentrations of reactants and products. In other words, do the concentrations of reactants and products affect the equilibrium constant if it's for the same reaction? Hopefully this makes sense

Aaina 2D

Hi! In general, whichever reaction (forward or reverse) is more favored is the one leading to more stability. So if the forward reaction is favored (large K value), the products are favored - more products are produced. This means that products have higher stability, because reactions tend to move i...

Aaina 2D

Hi! So your first 2 steps are correct regarding the balanced chemical equation and the setup for the reaction quotient. However, the concentrations that you're plugging in are incorrect. Notice how the moles are given, but not mol/L (which is your concentration and what you need to find the reaction...

Aaina 2D

Hi! If a reaction is at equilibrium with a given equilibrium constant, why is it that one side is favored over the other? In other words, why are the products sometimes favored over the reactants and vice versa if the reaction is at equilibrium (forward and reverse reactions have equal rates)?

Aaina 2D

Hi! To add on to the above responses, if the equilibrium concentration for a reactant or a product isn't given, the equilibrium constant (Kc) would be given instead. This way, using the products/reactants formula, you could set it equal to the equilibrium constant and solve for the unknown reactant ...

Aaina 2D

Hi! Like many of us, I will be spending time at home with just my family this winter. I'll probably catch up on netflix, and just relax before winter quarter begins!

Aaina 2D

Hi! You add 'ate' to the end of the TM cation when the overall charge on the complex is negative. For example, in this case, [Co (NH3)6] 3+ is just hexaammine cobalt (III) because the overall charge of the coordination sphere (complex) is positive. However, for [Co (CN)5 (OH2)] 2- you add 'ate' to t...

Aaina 2D

Also, one thing to know about diatomic molecules are that they are NOT polar.

Aaina 2D

Hi! For Week 8 Sapling Number 18, why is it that the hydrogen atoms are in the same plane for H2CCH2 and H2CCCCH2 but not for H2CCCH2?

Aaina 2D

Hi! The question here is how many protons could this base (thymine) accept? I thought we needed to count the lone pairs (2 on each oxygen and 1 on each nitrogen), giving a total of 6 sites to accept protons. However, the solutions manual said only the lone pairs on the nitrogen could accept protons....

Aaina 2D

Hi! If you go on his website and go to the acid and base outline, there's a list of points we should know for the final.

Aaina 2D

Hi! What intermolecular forces are present between CO2 and H2O?

I know CO2 is a nonpolar molecule, and H2O is a polar molecule, so is there still hydrogen bonding present?

Aaina 2D

Hi! Could someone explain why AX2E3 (the trigonal bipyramidal electron arrangement) is linear? What effect do the equatorial lone pairs have on the central atom in this case?

Aaina 2D

Hi! Yes, if it was a titration involving a weak acid and a weak base, the equivalence or stoichiometric point is where moles acid = moles base.

Aaina 2D

Hi! Why is hybridization needed for compounds like NH3? I understand why it’s necessary for carbon (because carbon makes 4 bonds, but there are only two unpaired electrons if unhybridized) but not for elements like nitrogen. Could someone please explain this?

Aaina 2D

Hi! Oxalate is a polydentate ligand, and therefore the prefix 'bis' is used instead 'di'. Other examples of polydentate ligands to watch out for are ethylenediamine (en), diethylenetriamine (dien), and ethylenediaminetetraacetato (edta). Bis, tris, tetrakis, etc. prefixes would be used for these.

Aaina 2D

Hi! Do different orbitals in the same atomic subshell (s,p,d,f) have the same energy or different? For example, does the orbital 2px have the same energy as 2py and 2pz orbitals?

Aaina 2D

You can determine if a species is acidic or basic by looking at whether it donates a proton or accepts a proton. If it donates a proton, then it’s acidic. If it accepts a proton, then it’s basic. Amphoteric species are ones that can react with both acids and bases depending on what other species th...

Aaina 2D

A chelate is formed when the ligand is polydentate. In addition to this, chelates are formed when more than one atom in the ligand have a lone pair and sigma bond on them, so that they can rotate and form a ring structure to bind to the metal cation in more than one place. Common examples of chelat...

Aaina 2D

In simple terms, a Bronstead acid is a proton donor (gives off a H+), and a lewis acid is an electron pair acceptor.

Aaina 2D

Hi! The way I look at it is if the polydentate ligand has more than one atoms with a lone pair AND a single bond, it can form a chelate.

Aaina 2D

Hi! In the image below, question 2E 1, could someone please explain why the 2nd model may have lone pairs?

Aaina 2D

I agree. Sodium is a cation that comes before the coordination complex (the bracket)

Aaina 2D

Hey! Just to add on, the coordination number represents the number of bonds that are formed by a specific cation, or transition metal. With this, we can use this information and the information we know about the shapes of molecules (i.e. coordination # of 6 being octahedral, # 4 being square planar...

Aaina 2D

Hi! These particular transition metals interact and bind with amino acids or other solutions in the active site of an enzyme. Hope this helps :)

Aaina 2D

Natallie K 3B wrote:But then why is trigonal bipyramidal not an answer?

This is because there are 5 bonds involved in trigonal bipyramidal. Hope this helps :)

Aaina 2D

I believe that the order of transition metals does not matter, but what's important is that the first row transition metals have unique biological functions.

Aaina 2D

Hi! When there is more than one ligand, you place them in alphabetical order. In today's lecture for example, [Co(NH3)5Cl], it would be penta ammine chloro cobalt (II) with ammine being before chloro.
Hope this helps :)

Aaina 2D

Hi! In the sapling homework, there was a question asking to draw structures of a compound/molecule, one of them being a ring structure.

How do we know when molecules have ring structures?

Aaina 2D

Like above, there is no difference in OH2 and H2O. They are just written like that in order to show which atom is actually bonding to the cation. In this case, O (the negatively charged atom) is what's binding to the cation. Therefore, for the purpose of showing which cation and atom are bonding, H2...

Aaina 2D

In general, molecules that have resonance, would have delocalized pi bonds. Hope this makes sense :)

Aaina 2D

Hi! The electron geometry refers to the arrangement of electron density around the central atom. So, for example, if there are 3 bonding pairs and 2 lone pairs around the central atom, there are 5 regions of electron density around the central atom. This makes the electron geometry trigonal bipyrami...

Aaina 2D

Hi! What exactly are delocalized pi bonds?

I was doing the sapling homework, and the question was

Which of the species contains a delocalized π bond?
- CO2−3
- H2O
- O3
- HCN

Could someone please explain what they are?

Aaina 2D

Hi! I believe when given a radical, the atom which is the most electronegative gets the full octet. So if you are deciding between giving one electron to which atom, go with the most electronegative one. This is because their pulling power of electrons is stronger, so therefore, the more electronega...

Aaina 2D

I believe discussions on Thursday and Friday will be cancelled this week.

Aaina 2D

To my understanding, pi bonds can't rotate or they will break. Sigma bonds, however, are able to rotate without breaking, so that may be another reason/example of sigma bonds being stronger. I agree with this statement. To add on, sigma bonds are stronger than pi bonds, because there is a bigger ov...

Aaina 2D

Hi! Like all the answers above, we leave the lone pair of electrons on N to minimize formal charge. When nitrogen is bonded to 3 atoms and has 1 lone pair of e-, the formal charge is zero. This makes the molecule more stable by minimizing energy.

Aaina 2D

Hi! The VSPER model is not on midterm 2. It goes till the end of outline 3 - chemical bonds. In the textbook, it's sections 1D to 1F, 2A-2D, and 3F.4 - 3F.5.

Aaina 2D

Hi! In the homework question below, I don't understand why the answer is CHI3. Could someone please explain?

3F5 - Suggest, giving reasons, which substance in each of the following pairs is likely to have the higher normal melting point (Lewis structures may help your arguments): c) CHI3 or CHF3

Aaina 2D

Hi! The double bond can be from Cl to the N atom, but this is unfavorable. If you double bond the N to the Cl, there are more formal charges present in the molecule ClNO2: Cl would have a +1 charge, N +1, and both O's would have a -1 charge. However, when double bonded to the oxygen, Cl has no charg...

Aaina 2D

Hi! I came across a problem where it asked me to identify which reactant is a lewis acid and which reactant is a lewis base.

CH3NH2 + CH3Br → Br- + (CH3)2NH2+

From this equation, how do we know which reactant is a lewis acid/base?

Aaina 2D

Hi! Polarizing power is an atom's ability to pull electrons (distort e- in other atoms), so cations are the ones that tend to have high polarizing power because their positive charge attracts negative electrons. There are two factors to remember when it comes to polarizing power: charge and size. Th...

Aaina 2D

Yes! The sum of all formal charges should equal the overall charge of that compound.

Aaina 2D

Hi! When it comes to expanded octets, do only central atoms have expanded octets, or can the surrounding atoms have expanded octets too? What I mean is, for example, ICl5 (iodine pentachloride) has 42e-. When drawing the structure, can the Cl's surrounding the central atom (I) have expanded octets l...

Aaina 2D

Adding on to what Megan said, the hydrogen in hydrogen bonds are only covalently bonded to an N, O, F, and that creates a dipole-dipole interaction with another electronegative atom. So really, there's only 1 bond, and the other is an attraction. Hope this helps!

Aaina 2D

Hi! When asked to write the ground-state electron configuration for an atom, do we write the noble gas configuration, or the full configuration? What I mean is, for example, the e- configuration for carbon is 1s^2 2s^2 2p^2, but can also be written as [He]2s^2 2p^2. Which one are we supposed to write?

Aaina 2D

Hi! Going off of what everyone said above, to determine the value of valence electrons, the e- present in the outermost shell, I look at the group numbers. While counting groups, I skip the group of transition metals, so Li is in group 1, Be in 2, B in 3, and C in group 4. Since carbon is present in...

Aaina 2D

Hi! While looking through the outline of the Quantum World unit, I came across this point: • Describe the interpretation of atomic orbitals in terms of probability. I am not sure what this means and what it is referring to. Could someone please explain? I am confused on how orbitals are related to p...

Aaina 2D

Hi! All ionic bonds have some covalent character, because in reality, the electron density around the two atoms is shared. And sharing = covalent.

Aaina 2D

To add on to all the answers above, usually, the elements P, S, and Cl have expanded octets. Also, I asked a TA about the limit to expanded octets, and she said we would usually see up to 14 electrons in total within an expanded octet.

Aaina 2D

Hi!
Yes. In general, ionic bonds are referred to as salts, and covalent bonds are referred to as molecules.

Aaina 2D

Also, since fluorine only requires one more electron to make it stable (a noble gas configuration), it is highly reactive, and therefore attracts electrons very easily.

Aaina 2D

I believe the chemical formulas will be given to you usually, but like above, anything ending with 'ate' suggests 3 oxygens (nitrate: NO3-), and anything ending with 'ite' suggests 2 oxygens (Nitrite: NO2-). Hope this helps!

Aaina 2D

Why do delocalized electrons (from resonance structures) lead to an increase in stability of atoms?

Aaina 2D

Why is the atom with the lower ionization energy the central atom? And are all other atoms bonded to the central atom?

Aaina 2D

Ionization energy and electronegativity are synonymous, so they have the same periodic trends, so I believe you can use either The above statement is true - they are synonymous. If you think about it, oxygen is very electronegative (high tendency to attract electrons), and therefore it has a higher...

Aaina 2D

What are delocalized electrons? Dr. Lavelle mentions it in his lecture when he explains resonance, and how those structures have delocalized electrons, but I'm not really sure what they mean.

Aaina 2D

Like the statements above, the number of electrons for an element is equivalent to its atomic number. To find the number of valence electrons, however, you would need to count the number of electrons in an elements' outermost shell. For example, in Magnesium, the number of electrons is = to its atom...

Aaina 2D

Hi! The side doesn't matter as long as you have the correct number of electrons for each element.

Aaina 2D

You use brackets for structures that have an overall charge. Therefore, you will see them when drawing cations and anions' lewis structures.

Aaina 2D

Hi! Electron affinity is basically the opposite of ionization energy. It's the energy released when electrons are added to a gas phase atom. It's not as periodic, but there's usually high e- affinity for elements in the top right of the periodic table. Halogens (group 17) have high e- affinities. In...

Aaina 2D

Once given the wavelength, you should be able to figure out whether it belongs to the Lyman or Balmer series. For example, if the wavelength belongs to one from the visible light spectrum, then it is the Balmer series, meaning n2 should be 2. If in Lyman's series, n2 is 1 After doing what was menti...

Aaina 2D

Polar molecules have an unequal sharing of electrons. For example, in the molecule H2O, O is more electronegative than H, making it attract more electrons. This gives them a partial negative charge, and hydrogen (H) a partial positive charge. The e- sharing is unequal, and therefore the molecule is ...

Aaina 2D

The energy does not get absorbed if it's less than the work function - it just passes through the metal

Aaina 2D

In general, when the question asks to calculate the number of moles of atoms or moles of photons, do you multiply by Avagadro's constant or divide by it?

Aaina 2D

Could someone please explain this question? Practitioners of the branch of alternative medicine known as homeopathy claim that very dilute solutions of substances can have an effect. Is the claim plausible? To explore this question, suppose that you prepare a solution of a supposedly active substanc...

Aaina 2D

Hi! So first, I would write the combustion reaction out: CxHyOz + O2 --> CO2 + H2O Once you have this, you're given the mass of CO2 and H2O produced, so the first thing you should always do is convert to moles. For CO2, it will be 1.2073g divided by its molar mass (44.01g/mol), which will give you t...

Aaina 2D

Hi! So once you got the work function - the energy to remove an electron - you take the total energy of photons, and divide it by the work function value: 7.4e-7/2.726e-19

This gives you the answer of 2.71e12 electrons ejected. Hope this helped!

Aaina 2D

In the photoelectric effect, if the energy of the photon is less than the threshold energy (work function), no electrons are ejected. Does that mean the photon just passes through the metal or does it still get absorbed with no electron ejection?

Aaina 2D

Hi! Spectral lines are basically showing electron energy levels - each line represents an energy transition of an electron. Hope this helps!

Aaina 2D

I believe the mass of electrons will be given on the formula sheet, and the same goes for the mass of protons and neutrons (if not on the formula sheet, it will be given with the question)!

Aaina 2D

In his lecture, Dr. Lavelle mentions that electrons have discrete energies with only certain wavelengths allowed inside an atom. What exactly does this mean?

Aaina 2D

I would say anything smaller than the wavelength 10^-15 would be hard to detect, making the wavelike properties unmeasurable.

Aaina 2D

Just like said above, you count the number of lines the electron can be de-excited to. So in this case, for example, from n= 5 to n = 4 is 1 line. However, since it doesn't give you a specific energy level to which the electron drops, count till the lowest energy level, which is n=1. n =5 to n =4 is...

Aaina 2D

For the frequency equation, v = -R [(1/n1^2)-(1/n2^2)], I was confused as to what n1 and n2 were.

So, for example, if an electron is emitting energy from energy level n= 5 to n= 2, what is n1 and n2 respectively in the frequency equation above? Which energy level is n1 and which energy is n2?

Aaina 2D

Hi! So the first part of the problem, you're given the frequency of the photons, and therefore, you can find the energy of the photons using the E = hv equation. Once you have the e(photon), and since you're given the kinetic energy, you can solve for the energy to remove an electron using the equat...

Aaina 2D

Hi! I'm not fully sure if this is right, but I think the only way different compounds can have the same molecular formulas is by being an isomer. Isomers have the same molecular formula but different structural formulas, so the atoms are the same, but the order they're in are arranged differently. T...

Aaina 2D

Hi! So we know the equation E(photon) - E(energy to remove e-) = kinetic energy of emitted e-. Therefore, if we solve for the energy of the photon and the energy required to remove the electron, we can find the kinetic energy. To solve for the energy needed to emit/remove an electron, you can use th...

Aaina 2D

Hi! I believe if light doesn't have enough energy to eject an electron, it just passes through the metal and is not absorbed.

Aaina 2D

Hi! An increase in light intensity doesn't increase the energy of light, but it increases the number of photons ejected. It is relative to the amplitude of the wave. And increasing the frequency indeed increases the energy of light, and this can be seen through the equation - E = hv, h being Planck'...

Aaina 2D

Hi! So for part B, there's no formula specific to incident light. Rather, you just manipulate the E = hv formula: E = hv = hc/wavelength

This way, once you find the energy of the incident light, you can use the formula above to find its wavelength.

Hope this helps!

Aaina 2D

I've heard that strategy too, and it typically always works. For me, in general, I find it the easiest to start with an element that's not H and O - I would balance all the other elements first. That being said, there's no specific order, but some people find it easier to leave O and H for last.

Aaina 2D

Hi! So to find the moles of each product, you need to first find the limiting reactant. Once you find the limiting reactant, you use its (the limiting reactant's) number of moles and the molar ratio to determine the number of moles of each product. To find the moles remaining from the excess reactan...

Aaina 2D

When looking for limiting reactants, what happens if the ratio of calculated moles is equal to the molar ratio in the balanced equation? Would that mean there's no limiting reactant? And if that were the case, which reactant would determine the amount of product formed?

Aaina 2D

In this question, it's asking you to find the molar mass of the sulfide (of the metal) if it had one. So first, you would find the molar mass of (OH)2, and subtract it from the molar mass of M(OH)2 (74.10 g/mol). This will give you the molar mass of M, and then using the periodic table, you find tha...

Aaina 2D

In the Fundamental review problems hw E9, the question starts with

"Epsom salts consist of magnesium sulfate heptahydrate. Write its formula."

I'm not sure how to write a formula with heptahydrate added. I know it's first MgSO4, but I don't know what comes next.

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