CHEMISTRY COMMUNITY

Chem_Mod

Question: For a pH of 3, would lactic acid or trichloroacetic acid be used? It seems like lactic acid is a better answer because the pKa is 3.08 which is closer to 3 than 2.85.

Chem_Mod

Answer: The amount of H3O+ changes for this reason: You add a strong base, so you are right, the only direct change from that is the change in concentration of acetic acid and acetate. This essentially means you are no longer at K, you are at Q now (changing the concentrations does that to you), wh...

Chem_Mod

Question: To calculate the pH of an acidic buffer solution to which some strong base has been added, how is the concentration of hydronium ions expected to change (resulting in a change in pH) when it seems the only concentrations that are changing are the acid (used up by the hydroxide ions) and c...

Chem_Mod

Answer: For this question it seems as if that would be okay since the change in x is so small that the H3O+ concentration doesn't change significantly. But you should always set up the ice box for this type of problem because you will not necessarily know if x will be so small that you can disregar...

Chem_Mod

Question: It asks for us to calculate the change in pH that occurs with the addition of .65 x 10^-6 mol H3O+ to sample. Instead of creating another ICE diagram, can we calculate the new moles of H3O+ in the reaction, find its molarity using the new number of moles, and then take the negative log of...

Chem_Mod

Answer: It depends on the expression K. If K = x^2/(1-x), x can be ignored in the (1-x) in order to simplify the calculation if K < 10^-5. In this case, we can neglect the x in 1-x and left with K = x^2.

Chem_Mod

Question: If K is less than 10^-5, can we neglect x even if the equation to be solved is quadratic and not cubic?

Chem_Mod

Answer: The whole Ka or Kb being 10^-5 or less is just a very general rule of thumb that tells you if you should even bother to assume the 5% rule (because if you do, and your x is NOT less than 5% of the initial concentration, you have to use the quadratic). The approximation (a.k.a. the 5% rule) ...

Chem_Mod

Question: When is it okay to approximate when finding the equilibrium concentrations of acids and bases by assuming that [x] is negligible? Is it when Ka or Kb is 10^-5 or less?

Chem_Mod

Answer: In the Henderson-Hasselbach equation, you SHOULD put in the molarity, but since the total volume is the same for both the base and the acid (they are sitting in the same solution, so they have the same total volume of solution), the volumes will cancel. For example say the total volume is 1...

Chem_Mod

Question: Even though we technically CAN use (moles base)/(moles acid) in the Henderson-Hasselbalch equation, is it the same as using molarity?

Chem_Mod

Answer: A buffer can resist pH change even with the addition of small amounts of strong acid or base (i.e. acids/bases that are not part of your buffer system). a buffer can effectively "absorb" added acid with little change in pH when [A-]/[HA] is at least 0.10. This is yet another way t...

Chem_Mod

Question: Why are buffers used? Isn't the addition of small amounts of acids or bases contradict the purpose of resisting a change of pH?

Chem_Mod

Answer: No, extra deprotonation steps just give the buffer more ranges in which to be used. Nonpolar solvents like hexane and toluene are not effective as proton donors/acceptors, but there are plenty of other polar (protic and aprotic) solvents that have acid/base reactivity

Chem_Mod

Question: Does the number of deprotonation steps in solution make a given buffer inherently stronger, i.e. a H3PO4 & Na3PO4 buffer or H2CO3 & Na2CO3? Also, do these steps always require water, or can other solvents be used to achieve the same effect?

Chem_Mod

Answer: Yes, it would take the form of pOH = pKb + log ([acid] / [base]).

Chem_Mod

Question: Can the Hendeson-Hasselbach equation be written for pOH instead of pH?

Chem_Mod

Answer: Salts work well to buffer the pH of solution because you pick a specific salt wisely. In the generic equation HA --> H+ + A-, you pick your salt to contain the cation A-. Thus, in a solution of this salt and HA, due to the high relative concentrations of HA and A-, the solution is rather ro...

Chem_Mod

Question: How do salts play a role in a buffer?

Chem_Mod

Answer: At pH 12 the solution is alkaline and the H3PO4 has already lost 2 or 3 H+ to form Na2HPO4 and Na3PO4. These are the species in solution at pH 12.

Chem_Mod

Question: When determining the pH values of a buffer, how come the pH value listed for the compound H3PO4 is 12, but the species NaHPO4 added with Na3PO4 also have a pH of 12. Why do these species only have one H atom, instead of 3 in H3PO4.

Chem_Mod

Answer: Using the Henderson-Hasselbalch equation you can determine the ratio of acid to base. This is equal to the ratio of moles of acid and base. Given the known moles of the acid you can then determine the moles of base necessary and calculate the volume necessary to deliver that many moles of b...

Chem_Mod

Question: How can we find the volume of a solution added using the Henderson-Hasselbalch equation?

Chem_Mod

Answer: The Henderson-Hasselbalch equation is used under the condition of weak acid and a salt or a weak base and a salt. Both of these are buffer solutions.

Chem_Mod

Question: Under what conditions can we use the Henderson equation to calculate the pH if we're not told in the question that there is a buffer involved? How do we know if a solution contains a buffer or not?

Chem_Mod

F- is an exception in that it is the only halogen that affects the pH of a solution. If one thinks about the series H-F, H--Cl, H---Br, H----I and recalls that H-F is the weakest acid (strongest bond) then it makes sense that F- will pick up H+ from water to form H-F and OH- and hence raise the solu...

Chem_Mod

K+ is not an acid and Br- is not a base, so the solution is neutral. But KF is basic, why is it not also neutral since they both have K+ and Br and F are in the same group? Also for compounds such as AlCl3 and Cu(NO3)2, how do I determine that it is Al(H2O)6 or Cu(H2O)6 when I am writing out the che...

Chem_Mod

base + acid ---> salt + water NaOH + CH3COOH -----> CH3COO-Na+ + H2O

Chem_Mod

How do you write the reaction for NaOH + CH3HOOH?

Chem_Mod

Because H2SO3 has two protons, 2 hydroxides are required to neutralize it, yielding the 2:1 ratio of NaOH:H2SO3 needed.

Chem_Mod

For a reaction of H2SO3 and NaOH, how many moles of NaOH are needed to neutralize it?

Chem_Mod

Use the Henderson-Hasselbach equation, pH = pKa + log([base]/[acid]). In the case of these species, H2PO3- + H2O -> HPO3(2-) + H30+, thus the dihydrogen species is the acid, and the monohydrogen specie is the base. Given that, in the Henderson-Hasselbach equation the pKa of dihydrogen phosphate must...

Chem_Mod

How do we differentiate to use the pKa for potassium dihydrogen phosphate and not the pKa for potassium hydrogen phosphate?

Chem_Mod

Remember that phosphoric acid is a triprotic acid (H3PO4). Ka1 refers to the reaction that removes the first hydrogen: H3PO4 + H2O <-> H2PO4- + H3O+. Ka2 refers to the reaction that removes the second hydrogen: H2PO4- + H2O <-> HPO4-2 + H3O+. Ka3 refers to removing the last hydrogen: HPO4-2 + H2O <-...

Chem_Mod

How do you find the value for pKa3 for phosphoric acid?

Chem_Mod

Answer: The relationship between pKa and pH is given by the Henderson-Hasselbach equation. Acid strength increases with decreasing pKa. A strong acid is not represented with an equilibrium constant because it is assumed to dissociate completely (Ka is "infinite," if you want). Weak acid d...

Chem_Mod

Question: Is there any relation between pKa and the pH or pOH? The smaller the pKa, the stronger the acid. The lower the pH, the stronger the acid. But why are certain acids with very small pKa (ex trichloroacetic) still considered weak acids?

Chem_Mod

Answer: When you have strong acids and bases that dissociate completely, you can directly calculate the pH (pOH) without having to do an ICE box. You wouldn't use Ka or Kb for strong acids and bases, respectively, because dissociation is assumed to be 100%.

Chem_Mod

Question: When we calculate the the H3O+ and OH- concentrations for strong acids and bases, can we use KaKb = Kw, because these acids/bases dissociate completely? As opposed to weak acids and bases when we have to use the ICE box?

Chem_Mod

Answer: Neutral water has 10-7 M H3O+. If one calculates that a compound when added to neutral water produces H3O+ (equivalent to a concentration of 10-10 M H3O+) then the compound is hardly increasing the H3O+concentration above 10-7 M. In other words, add 10-10 M H3O+ + 10-7 M H3O+ and then take ...

Chem_Mod

Question: I was confused about the last statement you made at the end of class: "If [H3O+] < 10-7, then the solution is considered neutral because we know that as a result of autoprotolysis they are 10-7 M H3O+." I am have trouble understanding how a solution would be neutral if [H3O+] is...

Chem_Mod

Answer: Correct! Although, all chemical reactions are reversible, so a double arrow is never incorrect.

Chem_Mod

Question: For a strong acid and strong base reaction, there is only one arrow representing that all of the reactants are converted to products, right? So, for weak acids and bases that do not dissociate completely, the double arrows are used, right?

Chem_Mod

Really strong acid/base has a Ka/Kb larger than 1. Although I would consider a strong acid/base with a Ka/Kb larger than 10^-3.

Chem_Mod

For the section on weak acids and bases, if only Ka or Kb are given, is there a way to tell whether the acid/base will be weak or strong?

Chem_Mod

All your statements are correct.

Chem_Mod

I wanted to clarify the concept of Ka (aciditiy constant) and Kb (basicity constant). We know that when Ka is a large number, such as 10-1, then it is a stronger acid because it has greater [H3O+]. So if Kb is also a large number such as 10-1, then it should also be a stronger base compared to a Kb ...

Chem_Mod

Since carbonic acid, H2CO3 has Ka1 = 4.3 x 10^-7, it is a weak acid.

Chem_Mod

Are all carbonic acids generally considered to be weak?

Chem_Mod

No, it is applicable to situations in which water is the solvent. Kw is always constant for a given temperature, and that is why we can calculate pH upon addition of acid or base in the first place.

Chem_Mod

Does the equation Kw = [H3O+][OH-] = 1.0 x 10-14 only apply to pure water?

Chem_Mod

You are confused between pH and pKa which are very different. pKa tells the degree of dissociation of an acid. pKa= -logKa and Ka = [H3O+]*[A-]/[HA]. The smaller the the pKa, the larger the Ka meaning the acid is strongly dissociated into H3O+. So indeed HClO is a strong acid compare to HBrO (pKa = ...

Chem_Mod

HClO which has a pKa of 7.53. How is it an acid if its pH is greater than 7? Is it dependent on whether is a proton donor rather than the pH?

Chem_Mod

In the pH range of ~6-8, the hydronium ion concentration provided by the weak acid is comparable to that of the autoprolysis of water. Because of this, the latter cannot be ignored because neither one of the hydronium ion sources is dominant enough to ignore the other. To take into account the autop...

Chem_Mod

Why must autoprotolysis need to be taken into account if the pH is greater than 6?

Chem_Mod

If you have a weak base in soln then use Kb. If a weak acid then use Ka. Remember you can always convert Ka to Kb and Kb to Ka. Ka x Kb = 10^-14

Chem_Mod

How do I know whether I am supposed to use the Ka or the Kb?

Chem_Mod

Usually Kb is used if the initial species are bases and Ka when your initial species are acids.

Chem_Mod

How do you tell when to use Ka and Kb when solving problems?

Chem_Mod

HCO3- (known as bicarbonate) is the conjugate base of H2CO3, a weak acid, and the conjugate acid of the carbonate ion. HCO3- acts as a base when mixed with a compound that is more acidic than itself (larger Ka) and as an acid when mixed with a compound that is more basic than itself (smaller Ka). Ac...

Chem_Mod

It seems as if HCO3- can act as both a base and an acid. How do you tell which it is?

Chem_Mod

HF is a weak acid, so F- from KF will pick up a proton from water to produce OH-.

Chem_Mod

The pH for the KF solution is above 7, but the pH for KBr is neutral. I don't understand why they're so different.

Chem_Mod

It is a sliding scale for the strength of acids and the strength of the conjugate base. The equation Kw=Ka*Kb tells us what the books says that "the stronger the acid the weaker the conjugate base", and vice versa. However, this doesn't exclude the fact that weak acids will have weak conju...

Chem_Mod

How does HF, a weak acid, have F- as a weak base?

Chem_Mod

Because F- is the anion of a weak acid (remember that HF is NOT a strong acid like the rest of the hydrohalic acids). The conjugate base of a weak acid is in fact basic in solution, raising the pH above 7. Both HBr and KOH are strong acids and bases, respectively. Conversely, Br- and K+ must be very...

Chem_Mod

Anions of strong acids are such weak bases that they have no significant effect on the pH of a solution and are considered basic in water, why does KF have a pH greater than 7 and not a pH equal to 7? Why is the pH 7 and not above 7 for KBr?

Chem_Mod

If you draw the Lewis structure for the conjugate base of HClO4 which is ClO4-. You will soon see that the anion is greatly stabilized by many resonance structures. Second, remember that O is more electronegative then Cl, so all O=Cl bonds would pull the electron densities away from the "-"...

Chem_Mod

How do highly electronegative atoms in a polyatomic conjugate base (like trichloroacetate) create a "chain effect" that diffuses the net charge and makes it more stable? Is this why a higher oxidation number for the central atom of an oxoacid (e.g. HClO4) makes it a stronger acid when depr...

Chem_Mod

No. If you have a strong acid, that means that it is totally deprotonated in solution. A strong base would be completely protonated in solution-- so the conjugate of either would have to be weak or else, by definition, the conjugate acid or base would completely ionize again to form the original aci...

Chem_Mod

If there is a strong acid, then does that mean it has a strong or weak conjugate base? And how about for the strong base ?

Chem_Mod

The Sb has an empty d-orbital that can accept an electron pair.

Chem_Mod

Why is SbF5 considered a lewis acid?

Chem_Mod

The Sb has an empty d-orbital while the F lone pairs are tightly held.

Chem_Mod

I have a question about SbF5 as a lewis acid/base. What about all the lone pairs on the F atoms? Couldn't these be donated so that SbF(5) would act as a base? Or is it because of the electronegativity of F that that won't happen?

Chem_Mod

Your reasoning is correct but there are more factors that determine e- pair availability and it turns out that H20 is more basic. Clearly their is a balance between the larger negative charge and the availability of the lone pairs. In the case of H20 versus H2S, the larger negative charge favors H20...

Chem_Mod

According to the solution manual H20 is more basic or is a stronger Lewis base than H2S because oxygen is more electronegative than S and has a higher partial negative charge. But if O is more electronegative than S, wouldn't that mean that O has a greater pulling force of electrons than S? And if t...

Chem_Mod

A Lewis base must have a lone pair of electrons avialable to pick up a proton. A Lewis acid must be able to accept a pair of electrons. Since boron in BF3 has 6 valence electrons it can accept 2 more giving it an octet. In many cases it is helpful to know the Lewis structure or the electron configur...

Chem_Mod

I am unsure how to distinguish a Lewis Acid from a lewis base. For example, how do we know that BF3 is a Lewis acid?

Chem_Mod

Al3+ forms a complex with water as ligands and Al3+ is a Lewis acid; therefore, AlCl3 gets substituted by Al(H2O)6.

Chem_Mod

Why did AlCl3 get substituted by Al(H2O)6?

Chem_Mod

Small, highly charged cations (Fe3+, Al3+, etc.,) form weekly acidic solutions by interacting with water and the water releasing a proton. When Al3+ is disolved in water, it becomes hydrated, and the hydrated form acts as a Bronsted-Lowry acid.

Chem_Mod

How is AlCl3 acidic?

Chem_Mod

Pyridine is a base, it is basic because of the nitrogen on it that can donate its electron pair. H+ and AlCl3 are acidic because H+ and Al(3+) are electron deficient.

Chem_Mod

Why are pyridine, H+, and AlCl3 considered Lewis Acids?

Chem_Mod

Electronegativity can be useful, as the more electronegative a substance is, the stronger the acid it is. Another useful tool is by considering either its lewis structure or its electron configuration and determining if it is likely to gain or lose electrons.

Chem_Mod

How do you identify whether species are acidic or basic if presented with a molecule that doesn't have the common H- for acids and OH- for bases?

Chem_Mod

It is amphoteric.

Chem_Mod

Should Bi2O2 be basic or amphoteric?

Chem_Mod

These compounds are amphiprotic, meaning that in water they can behave as both a proton donor or a proton acceptor (acid or a base). In general it is difficult to determine the pH of an amphiprotic compound (how to do that is discussed more in Section 10.15, which is not part of your assigned readin...

Chem_Mod

In water, should we assume HSO4- acts as an acid or a base. Also for HCO3-?

Chem_Mod

The formula is used when a polyprotic compound is amphiprotic (it acts as both an acid and a base). Because it can behave as both, it is not obvious if the pH will be acidic or basic. Noting that a high pKa means a small Ka value, having both high pKa1 and pKa2 values tells you that the polyprotic c...

Chem_Mod

Why is the formula pH=(1/2)(pKa1+pKa2) used?

Chem_Mod

Yes. Remember water can act as an acid or a base. In this case, water gives off proton, water is an acid.

Chem_Mod

In the reaction NH3 (aq) + H2O (l) --> NH4+(aq) + OH- (aq), NH3 is the weak base, but does that mean that H2O is considered an acid because it donates its proton to form OH-?

Chem_Mod

We covered metal oxides and hydroxides (e.g., NaOH) which are basic compounds. We also covered non-metal oxides (e.g., H2SO4, H3PO4, etc.,) which are acidic compounds. The metalloids (diagonal band between metals and nonmetals) form amphoteric compounds, that is they can act like an acid or base. So...

Chem_Mod

The book mentions that d-block metals form acidic, amphoteric, or basic oxides form depending on their oxidation state. Are they saying that particular d-block metals form amphoteric oxides, and if so, which ones?

Chem_Mod

In general, metal oxides (i.e. CaO) are basic while nonmetal oxides (i.e. CO2) are acidic. Oxides made with the elements known as metalloids (B, Si, Ge, As, Sb, Te, and Po) can be acidic or basic depending on their environment; in other words they are amphoteric. (However, this general trend cannot ...

Chem_Mod

How do we determine whether oxides are acidic, basic, or amphoteric?

Chem_Mod

No. Noble gases are mostly unreactive and do not act as bases.

Search found 16459 matches