Buffers

Which of the following substances (along with its corresponding salt) would be best suited for generating a buffer solution with a pH below 7? Select the correct answer below: CH3CO2H NH3 HCl none of the above

NH3

Which of the following will exhibit the greatest change in pH when 1.0 mL of 1M HCl is added to it? Select the correct answer below: an unbuffered aqueous solution an unbuffered solution that is 0.15M in NH3 100 mL of a buffer solution that is 0.15M in NH3 and 0.15M in NH4Cl 50 mL of a buffer solution that is 0.05M in NH3 and 0.15M in NH4Cl

an unbuffered aqueous solution

Which is a possible example of species that can comprise a buffer solution? Select the correct answer below: NH3 and H2O NH3 and NH4Cl NH3 and CH3NH2 CH3NH2 and NaNH2

NH3 and NH4Cl Ammonia and ammonium chloride are the only example of a weak base paired with its conjugate acid.

If a certain amount of strong acid is added to a buffer solution with pH 8, and the pH after the addition of the acid is 1.4, what can we say about the solution?

The buffer capacity has been exceeded. This represents a drastic change in pH, therefore the buffer capacity of the solution has been exceeded by a wide margin.

Which will cause the greatest change in the pH of a buffer solution ? Select the correct answer below: the addition of a quantity of an acid that is equal to half of the solution's buffer capacity the addition of a quantity of an acid that is equal to three fourths of the solution's buffer capacity the addition of a quantity of an acid that is equal to the solution's buffer capacity the addition of a quantity of an acid that is equal to one and a half times the solution's buffer capacity

the addition of a quantity of an acid that is equal to one and a half times the solution's buffer capacity

In order to generate a buffer solution with a pH below 7, what type of substance should be used? Select the correct answer below: weak acid and a salt containing its conjugate base strong acid and a salt containing its conjugate base strong base and a salt containing its conjugate acid None of the above

weak acid and a salt containing its conjugate base

A buffer solution is able to resist changes in pH:

when small amounts of acid or base are added

In a buffer solution made from acetic acid and the acetate ion, once the acetic acid concentration drops to 10% of the acetate ion, the pH will quickly shift by what amount? Select the correct answer below: 0.1 1 5 10

1 Assume that the initial pH is equal to the pKa. The Henderson-Hasselbalch equation is the following. pH=pKa+log[base][acid] Since the acid drops to 10% of the acetate ion, the equation can be written as follows. pH=pKa+log[100][10] pH=pKa+log(10) pH=pKa+1 The pH will climb 1 unit.

What will be the pH of a buffer solution with an acid (pKa4.9) that is exactly ten times as concentrated as its conjugate base?

pH=3.9 Given the Henderson-Hasselbalch equation: pH=pKa+log[A−][HA] We know the ratio of [HA] to [A−] is 10:1, and the pKa value, so we plug these into the Henderson-Hasselbalch equation: pH=4.9+log(110)=3.9 Notice that the ratio is exact, so the values in the ratio do not constrain the number of significant figures.

As one component of a buffer solution is consumed, the pH will change:

slowly then rapidly

Which buffer will have a pH of 4.19? Select the correct answer below: 1.0 MHOCH2CO2H(pKa=3.83)+1.0 MHOCH2CO−2 1.0 MCH3CHCO2H(pKa=3.08)+1.0 MCH3CHCO−2 1.0 MHCO2H(pKa=3.75)+1.0 MHCO−2 1.0 MC6H5CO2H(pKa=4.19)+1.0 MC6H5CO−2

1.0 MC6H5CO2H(pKa=4.19)+1.0 MC6H5CO−2 The Henderson-Hasselbalch equation can be used to determine the pH of a bufer. pH=pKa+log[A−][HA] In all of the buffers, the [A−]=[HA] so the equation simplifies to pH=pKa+log[A−][HA] pHpHpHpH=pKa+log1.01.0=pKa+log (1)=pKa+0=pKa Therefore, the buffer whose acid component has pKa=4.19 will have a pH=4.19 because the concentration of the buffer components is the same.

Which buffer will have the greatest buffer capacity? Select the correct answer below: 1.0 MNH3+1.0 MNH+4 0.5 MNH3+0.5 MNH+4 0.3 MNH3+0.3 MNH+4 0.1 MNH3+0.1 MNH+4

1.0 MNH3+1.0 MNH+4 Buffer capacity is a measure of a buffer's resistance to changes in pH. A greater buffer capacity means that a buffer can withstand changes in pH when larger amounts of strong acid or strong base are added. A smaller buffer capacity means that only small amounts of strong acid or base can be added before the pH of the buffer begins to change. The more concentrated the acidic and basic components of the buffer, the greater the buffer capacity. This is because larger amounts of added strong acid or strong base can react with the buffer components before the pH changes to a larger extent. The most concentrated buffer, 1.0 MNH3+1.0 MNH+4, will have the greatest capacity.

Which mixture is the best choice to produce a buffer with pH>7? 1.0 MNH3+0.1 MNH+4 1.0 MNH3+1.0 MNH+4 1.0 MCH3CO2H+1.0 MNaCH3CO2 1.0 MCH3CO2H+0.1 MNaCH3CO2

1.0 MNH3+1.0 MNH+4 The most effective buffer (the one having the best buffering action) is one where the acidic and basic components are present in roughly equal concentrations. A buffer where the components concentrations are both present at 1.0 M will be more effective than a buffer where one component has a concentration of 1.0 M and the other component has a concentration of 0.1 M. A weak acid and its salt will produce buffers with pH<7 and a weak base and its salt will produce buffers with pH>7. Since NH3 is a weak base and NH+4 is its salt, this combination will produce a buffer with pH>7.

A buffer solution has equal concentrations of acetic acid and the acetate ion and has a pH that is initially 4.74. What percentage must the acetic acid concentration decrease by to result in a pH increase of one unit?

10 Initially at pH 4.74, the pH of this buffer solution will rapidly climb by at least 1 pH unit if the acetic acid concentration dips below 10% of the acetate ion concentration.

Joanna is performing a reaction that generates a moderate amount of hydroxide. To mimic biological conditions (most bodily fluids in living organisms retain a relatively constant pH around 7), she knows that she needs to add a buffer to the reaction. Which of the following will MOST effectively neutralize the OH− ions produced by the reaction?

A buffer containing H3PO4 and H2PO4−, with a slight excess of H3PO4 The HCl/NaCl buffer is not a buffer at all, and thus will not resist changes in pH. Both of the ammonia/ammonium buffers described are basic; the one that contains an excess of ammonia is more so. Therefore, neither of these will be as effective at neutralizing hydroxide as the acidic H3PO4/H2PO4− buffer.

In a sample buffer solution, as the strength of the weak base decreases (i.e., as the pKb increases), what happens to the strength of the conjugate acid? As the strength of the base decreases, the conjugate acid increases in strength. A weak base has a weak conjugate acid. As the strength of the weak base decreases the strength of the conjugate acid decreases as well. It depends, more information is needed to determine the strength of the conjugate acid.

As the strength of the base decreases, the conjugate acid increases in strength. The strength of a base and its conjugate acid are inversely proportional to each other. As the weak base gets weaker (i.e., as the pKb increases), the conjugate base gets stronger.

Which of the following combinations would make the best buffer? Select the correct answer below: H3PO4 and NaOH NaH2PO4 and H2O NaOH and NaNO3 H3PO4 and NaH2PO4

H3PO4 and NaH2PO4 A buffer is a mixture of a weak acid or a weak base and the salt of its conjugate, such as H3PO4 and NaH2PO4.

Of the following, which are accurate statements regarding buffer capacity? If enough base is added to a buffer to react with all of the weak acid present, the buffer can no longer resist increases in pH. If enough acid is added to a buffer to react with all of the weak base present, the buffer can no longer resist decreases in pH. Greater concentrations of a weak base and its conjugate acid in a buffer lead to decreased buffer capacity. Lower concentrations of a weak acid and its conjugate base in a buffer lead to decreased buffer capacity.

If enough base is added to a buffer to react with all of the weak acid present, the buffer can no longer resist increases in pH. If enough acid is added to a buffer to react with all of the weak base present, the buffer can no longer resist decreases in pH. Lower concentrations of a weak acid and its conjugate base in a buffer lead to decreased buffer capacity.

Which pair, when equal concentrations are mixed, can act as a buffer? Select all that apply: NH3 and NH4Cl HCl and NaCl HF and KF H2O and NaOH

NH3 and NH4Cl HF and KF An acid base buffer consists of a weak conjugate acid base pair (either weak acid + conjugate base or weak base + conjugate acid). The acidic and basic components of the buffer must be capable of reacting with added base or acid, respectively, to minimize changes in pH upon addition of strong acid or base. NH3 is a weak base, and NH+4 is its conjugate acid, so this pair would form a buffer. The Cl− in NH4Cl is a spectator ion and does not react with water. HCl is a strong acid, so its conjugate base Cl− is a very weak base. It is such a weak base that it is considered a spectator ion. The ion Na+ is also considered a spectator ion. Since Cl− is unable to react with added strong acid, this pair would not form a buffer. HF is a weak acid, and F− is its conjugate base, so this pair would form a buffer. The K+ in KF is a spectator ion and does not react with water. H2O is negligible acid, so its conjugate base OH− is a strong base. Since this pair is not a weak conjugate acid base pair, this pair would not form a buffer.

If we add acid to a buffer containing NaCH3CH2COO and CH3CH2COOH, the acid will react with which of the following? Select the correct answer below: NaCH3CH2COO CH3CH2COOH Na+ No reaction will occur.

NaCH3CH2COO The H+ ions dissociated from the acid will react with the propanoate (CH3CH2COO−) ions to form more CH3CH2COOH and water, thereby neutralizing the acid with relatively little change in the pH of the solution.

When a small amount of a strong acid or a strong base is added to a buffer solution, the pH will: decrease if a strong acid is added increase if a strong base is added remain relatively constant it depends on the strength of the acid or base

Remain relatively constant Buffer solutions resist a change in pH when small amounts of a strong acid or a strong base are added. If a strong acid is added, the pH may decrease ever so slightly, while the addition of a strong base may increase the pH only very slightly.

If a certain amount of strong base is added to a buffer solution with pH of 5.98, and the pH after the addition of the base is 6.02 , what can be said about the solution? Select the correct answer below: The solution's buffer capacity has not yet been reached. The solution is at its buffer capacity. The solution's buffer capacity has been exceeded. The solution may be below or in excess of its buffer capacity, depending on the nature of the base.

The solution's buffer capacity has not yet been reached.

In a buffer solution made of acetic acid and sodium acetate, if a small amount of acid is added, the added acid will react with which of the following? Select the correct answer below: hydronium ions sodium ions acetic acid acetate ions

acetate ions The acetate ion is available to be protonated by any strong acid that is added to solution, as shown below. CH3COO−(aq)+H3O+(aq)⟶CH3COOH(aq)+H2O(l)

In a buffer solution made of acetic acid and sodium acetate, any base that is added will react primarily with which of the following? Select the correct answer below: sodium hydroxide acetic acid acetate

acetic acid When a base is added to a buffer, it will react with (and be neutralized) by the acid component of the buffer, which is acetic acid in this case.

Which of the following will exhibit the greatest change in pH when 1.0 mL of 1.5M NaOH is added to it? Select the correct answer below: 50 mL of a buffer solution that is 0.20M in CH3COOH and 0.20M in NaCH3COO 100 mL of a buffer solution that is 0.10M in CH3COOH and 0.10M in NaCH3COO an unbuffered solution that is 0.10M in CH3COOH an unbuffered aqueous solution that is 0.10M in NaCH3COO

an unbuffered aqueous solution that is 0.10M in NaCH3COO The buffered solutions can resist changes in pH, whereas the unbuffered solutions cannot. The acetic acid solution will be able to neutralize some of the base before having its pH elevated, but the water in the aqueous solution of NaCH3COO will immediately react with the strong base to form hydroxide ions, thus affecting the pH the most.

If the concentrations of a weak acid and its conjugate base are decreased from 0.5 M and 0.2 M, respectively, to 0.3 M and 0.04 M, the solution's buffer capacity will _________. increase decrease remain constant decrease then increase

decrease

The buffering ability of a solution will: diminish slowly until the buffer capacity is reached, then continue to diminish slowly. diminish rapidly until the buffer capacity is reached, then continue to diminish rapidly. diminish slowly until the buffer capacity is reached, then begin to diminish rapidly. never diminish.

diminish slowly until the buffer capacity is reached, then begin to diminish rapidly. The buffering ability of a solution will begin to diminish before the buffer capacity is reached, but relatively slowly. Once the capacity is reached, the solution no longer has any buffering ability whatsoever, so the pH will begin to change rapidly.

Enough acid or base has been added to a pH 5.5 buffer to raise the pH to 6.5. If that same amount of acid or base is added again, the pH will likely change by: less than one unit more than one unit precisely one unit impossible to tell

more than one unit After the initial rise in pH from 5.5 to 6.5, the solution is close to its buffer capacity. This means that the addition of more acid or base will change the pH much more dramatically after this point.