2 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 2
Buffer X is 0.10 M in CH₃COOH and 0.10 M in NaCH₃COO. Buffer Y is 1.0 M in CH₃COOH and 1.0 M in NaCH₃COO. Both are at 25°C. Which statement correctly compares the pH values of the two buffers?
Answer and reasoning
AThey are equal, because every buffer solution has a neutral pH A student who thinks every buffer is neutral picks this. The pH values are equal, but both are about 4.74, the pKa of acetic acid, not 7; they are equal because the ratio of the components is the same.
BThey are equal, because the ratio [CH₃COO⁻]/[CH₃COOH] is the same in bothCorrect pH = pKa + log([CH₃COO⁻]/[CH₃COOH]). Increasing both concentrations tenfold keeps the ratio at 1, so both buffers have pH = pKa; Y differs only in having the greater capacity.
CY's pH is lower, because Y contains a higher concentration of CH₃COOH A student who thinks more weak acid always means a lower pH picks this. Y also has ten times as much CH₃COO⁻, so the ratio [CH₃COO⁻]/[CH₃COOH], and the pH, are unchanged.
DX's pH is lower, because CH₃COOH ionizes to a greater extent in X A student who thinks a greater percent ionization in the more dilute buffer means more H₃O⁺ picks this. In a buffer, [H₃O⁺] = Ka[CH₃COOH]/[CH₃COO⁻], which is the same in X and Y because the ratio is the same.
Working No calculation needed. pH = pKa + log([CH₃COO⁻]/[CH₃COOH]); the ratio is 1 in both buffers, so both have pH = pKa = 4.74. Y has the greater capacity, but the same pH.
A buffer is made from CH₃COOH and NaCH₃COO. Which quantity sets the maximum amount of HCl that the buffer can neutralize before its pH begins to fall sharply?
Answer and reasoning
AThe amount of CH₃COOH present A student who thinks the acid component of a buffer neutralizes added acid picks this. CH₃COOH is a proton donor and does not react with H₃O⁺; it limits the buffer's capacity for added base.
BThe pH of the buffer itself A student who thinks capacity is set by the buffer's pH picks this. Buffers with the same pH can contain very different amounts of CH₃COO⁻ and so neutralize very different amounts of HCl.
CThe Ka of the acetic acid A student who thinks the strength of the weak acid sets the capacity picks this. Ka, with the ratio of the components, sets the pH; the amount of HCl neutralized depends on the moles of CH₃COO⁻.
DThe moles of CH₃COO⁻ presentCorrect Added H₃O⁺ reacts with CH₃COO⁻ in a 1:1 ratio, so the moles of CH₃COO⁻ in the buffer set how much HCl it can neutralize before the pH falls sharply.
Working No calculation. Added acid reacts with the conjugate base: CH₃COO⁻ + H₃O⁺ → CH₃COOH + H₂O, 1:1. Once the CH₃COO⁻ is used up, added H₃O⁺ remains and the pH falls sharply, so the quantity needed is the amount (moles) of CH₃COO⁻.
In preparation: 0 of 2 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
8.10.A.1 Buffer capacity Fix
Buffer capacity
The amount of added strong acid or strong base that a buffer can neutralize before its pH begins to change sharply; it is limited by the amounts of the buffer components available to react.
Effect of buffer concentration
Increasing the concentrations of both buffer components while keeping their ratio constant leaves the pH unchanged (the ratio [A⁻]/[HA] is the same) but increases the buffer capacity, because more of each component is available to react.
Students often think Buffer capacity is set by the buffer's pH, that is, by the ratio [A⁻]/[HA], so buffers with the same ratio have the same capacity whatever their concentrations. In fact No. The pH depends on the ratio [A⁻]/[HA], but the capacity depends on the amounts of HA and A⁻ available to react. A buffer with ten times the concentrations of both components has the same pH and ten times the capacity, for the same volume.
Students often think A buffer that contains a higher concentration of the weak acid has a lower pH, even when the conjugate base is increased in the same proportion. In fact No. Increasing both components in the same ratio leaves [A⁻]/[HA], and so the pH, unchanged; the extra conjugate base offsets the extra acid.
8.10.A.2 Capacity for added acid Fix
Capacity for added acid
Added strong acid is neutralized by the conjugate base, A⁻ + H₃O⁺ → HA + H₂O, so the amount of A⁻ in a buffer limits how much strong acid it can neutralize.
Capacity for added base
Added strong base is neutralized by the conjugate acid, HA + OH⁻ → A⁻ + H₂O, so the amount of HA in a buffer limits how much strong base it can neutralize.
Unequal buffer components
A buffer with more conjugate acid than conjugate base has a greater capacity for added base than for added acid; a buffer with more conjugate base than conjugate acid has a greater capacity for added acid than for added base.
Students often think The acid component, HA, neutralizes added acid and the base component, A⁻, neutralizes added base, so a buffer with more HA has the greater capacity for added acid. In fact The conjugate base, A⁻, which reacts with added H₃O⁺. The conjugate acid, HA, reacts with added OH⁻ and so limits the amount of strong base the buffer can neutralize.
Students often think A buffer with more of the protonated form, HA, has the higher pH. In fact No. With the same weak acid, the buffer with more HA than A⁻ has the lower pH: pH = pKa + log([A⁻]/[HA]) is below the pKa when [HA] > [A⁻].
4 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 4
The diagrams represent equal volumes of two buffer solutions, X and Y, made from the same weak acid, HA, and its sodium salt, NaA. Water molecules and Na⁺ ions are not shown. Which buffer can neutralize more added HCl before its pH falls sharply, and why?
Answer and reasoning
AY, because it contains more HA molecules to react with added H₃O⁺ A student who thinks the acid component of a buffer neutralizes added acid picks this. Y does neutralize more HCl, but because of its A⁻ ions; HA is a proton donor and does not react with H₃O⁺.
BNeither, because the ratio of A⁻ to HA is the same in X and Y A student who thinks capacity depends only on the ratio of the components picks this. The equal ratio gives the two buffers the same pH, but Y has three times as many A⁻ ions available to react.
CY, because it contains more A⁻ ions to react with added H₃O⁺Correct Added H₃O⁺ reacts with A⁻ to form HA. Both buffers have the same 1:1 ratio, and so the same pH, but Y has three times as many A⁻ ions in the same volume, so it can neutralize about three times as much HCl.
DX, because it has more water around each particle to absorb H₃O⁺ A student who thinks a buffer works by absorbing or diluting added H₃O⁺ picks this. Water does not remove H₃O⁺; the reaction with A⁻ does, and Y contains more A⁻.
Working No calculation. X: 2 HA, 2 A⁻; Y: 6 HA, 6 A⁻. Same ratio (1:1), so same pH. Added H₃O⁺ reacts with A⁻; Y has three times as many A⁻ in the same volume, so it can neutralize three times as much HCl.
Buffers P and Q each contain a total of 0.100 mol of the weak acid HA and its conjugate base A⁻ in 1.00 L. P contains more HA than A⁻, and Q contains more A⁻ than HA. The graph shows the pH of each buffer as 1.00 M NaOH(aq) is added; the curves are labeled 1 and 2. Which curve represents buffer P, and why?
Answer and reasoning
ACurve 1, because P has more HA molecules to neutralize the added OH⁻Correct Added OH⁻ reacts with HA, so the buffer with more HA neutralizes more NaOH before its pH rises sharply. Curve 1 rises sharply only near 80 mL, and it also starts at the lower pH expected when HA exceeds A⁻.
BCurve 1, because HA reacts only slightly with OH⁻, so P's pH rises slowly A student who thinks a weak acid reacts with only a small fraction of added OH⁻ picks this. P is curve 1, but its pH rises slowly because each OH⁻ added is consumed completely by HA, which is present in large amount.
CCurve 2, because P's smaller amount of A⁻ can react with less added OH⁻ A student who thinks the base component of a buffer neutralizes added base picks this. Added OH⁻ reacts with HA, not A⁻, so P, with more HA, neutralizes more NaOH: curve 1.
DCurve 2, because P's larger amount of HA gives it the higher starting pH A student who thinks more of the protonated form means a higher pH picks this. With more HA than A⁻, P's pH starts below the pKa, so P is the curve with the lower starting pH, curve 1.
Working No calculation. Added OH⁻ reacts with HA (HA + OH⁻ → A⁻ + H₂O), so the buffer with more HA neutralizes more NaOH before its pH rises sharply. Curve 1 starts lower (more HA → pH below pKa) and rises sharply only near 80 mL (0.080 mol HA); curve 2 starts higher and rises sharply near 20 mL (0.020 mol HA). P is curve 1.
A student prepares a buffer by mixing 50.0 mL of a CH₃COOH solution labeled 0.20 M with 50.0 mL of 0.20 M NaCH₃COO. Unknown to the student, the CH₃COOH solution is actually 0.10 M. Compared with the intended buffer, how does the amount of HCl that the actual buffer can neutralize before its pH falls sharply compare, and why?
Answer and reasoning
AIt is unchanged, because a buffer can neutralize any amount of added acid A student who thinks a buffer can neutralize unlimited acid picks this. Capacity is finite: once the 0.0100 mol of CH₃COO⁻ has reacted, further HCl lowers the pH sharply. The capacity is unchanged here only because the amount of CH₃COO⁻ is unchanged.
BIt is lower, because less CH₃COOH is present to react with added H₃O⁺ A student who thinks the acid component of a buffer neutralizes added acid picks this. CH₃COOH does not react with H₃O⁺; the smaller amount of CH₃COOH lowers the capacity for added base, not for added acid.
CIt is higher, because a more dilute CH₃COOH ionizes to a greater extent A student who thinks a more dilute weak acid produces more ions picks this. A larger fraction of a smaller amount ionizes, and in the buffer that ionization is negligible; the CH₃COO⁻ that neutralizes HCl comes from the salt, whose amount is unchanged.
DIt is unchanged, because the amount of CH₃COO⁻ in it is still the sameCorrect Added H₃O⁺ is neutralized by CH₃COO⁻, and the NaCH₃COO solution was the correct concentration, so the buffer contains the intended 0.0100 mol of CH₃COO⁻. The error halves the CH₃COOH, which lowers the capacity for added base, not for added acid.
Working No calculation of pH. Added HCl is neutralized by CH₃COO⁻; the NaCH₃COO solution was correct, so the actual buffer contains the intended 0.0100 mol of CH₃COO⁻. Its capacity for added HCl is unchanged (its capacity for added base is halved, and its pH is higher than intended).
A student wants to find out whether the buffer capacity of a CH₃COOH/CH₃COO⁻ buffer depends on the concentrations of its components. A pH meter, standardized 0.10 M HCl, burets and volumetric glassware are available. Which procedure is best suited to the question?
Answer and reasoning
APrepare 1:1 buffers at several concentrations; titrate 50.0 mL of each with HCl until the pH falls by 1 unitCorrect Only the concentration varies (the 1:1 ratio is fixed), and capacity is measured directly as the volume of HCl that equal volumes of the buffers neutralize before the pH falls by 1 unit.
BPrepare buffers that differ in both concentration and ratio; titrate 50.0 mL of each with HCl until the pH falls by 1 unit A student who thinks it is enough to vary the factor under study picks this. The ratio also differs, and it affects how much HCl each buffer can take before its pH falls, so the effect of concentration cannot be separated.
CPrepare 1:1 buffers at several concentrations and compare the pH that the meter reads for each one A student who thinks capacity is set by a buffer's pH picks this. Buffers with the same 1:1 ratio have the same pH whatever their concentrations, so measuring pH alone cannot show a difference in capacity.
DTake 10 mL, 25 mL and 50 mL samples from one buffer and compare the pH that the meter reads for each A student who confuses the amount of a solution with its concentration picks this. Samples taken from one buffer all have the same concentrations, ratio and pH, so this procedure varies nothing relevant to the question.
Working No calculation. The independent variable is concentration, so the ratio must be held constant; capacity is measured by how much HCl a fixed volume of each buffer neutralizes before its pH drops (e.g., by 1 unit). Measuring only the starting pH does not measure capacity; samples of one buffer have the same concentration.
Compiled from the AP Chemistry Course and Exam Description (effective Fall 2024) and our question bank · Specialist review in progress. How these pages are made · Free, no account
