/
Acid Base Equilibria and Buffers Review
Save to my account
Sign up
Acid Base Equilibria and Buffers Review
Acid Base Equilibria and Buffers Review
Study
1
Question
From the data given, which conjugate acid-base pairs are formed when methanoic acid is added to ethanoic acid in an equilibrium with two acid-base pairs? Label them A1,B1 and A2,B2.
Answer
A1: HCOOH ⇌ HCOO− + H+ ; B1: HCOO−; A2: CH3COOH ⇌ CH3COO− + H+ ; B2: CH3COO−.
2
Question
Calculate the pH of a 0.0800 M C2H5COOH solution using Ka data provided. Give to 2 d.p.
Answer
pH = 2.88.
3
Question
For a weak acid with Ka = 1.35×10−5 and initial concentration 0.0800 M, what is [H+] approximately? (Use approximation method)
Answer
[H+] ≈ 1.04×10−3 M.
4
Question
A buffer is prepared by mixing 400 cm3 of 0.204 M CH3COOH with 600 cm3 of CH3COONa. If the target pH is 4.00, what concentration of CH3COONa must be in the mixed solution? (Assume volumes add and Ka(CH3COOH)=1.78×10−5)
Answer
[CH3COONa] = 0.0300 M (approximately).
5
Question
What is the Henderson–Hasselbalch form for CH3COOH/CH3COO− buffer?
Answer
pH = pKa + log([CH3COO−]/[CH3COOH]).
6
Question
Write the conjugate acid-base pairs for the equilibrium of CH3SO2OH with water and indicate A1,B1 and A2,B2. (pKa = −1.90)
Answer
A1: CH3SO2OH; B1: H3O+ ; A2: CH3SO2O−; B2: H2O.
7
Question
Explain whether 1.0 M CH3SO2OH would have a lower pH than 1.0 M CH3COOH, based on pKa values. (Ethanoic acid pKa 4.76, methanesulfonic acid pKa −1.90)
Answer
No; CH3SO2OH is far stronger (lower pKa), so it yields lower pH.
8
Question
State the equilibrium equation showing CH3SO2OH reacting with water to form the conjugate base and hydronium. Label A1,B1 and A2,B2.
Answer
A1: CH3SO2OH; B1: H3O+ ; A2: CH3SO2O−; B2: H2O.
9
Question
Calculate pH of 0.0800 M C2H5COOH and give to 2 d.p. (Ka = 1.35×10−5)
Answer
pH = 2.88.
10
Question
During titration of 25.0 cm3 of 0.0800 M C2H5COOH with NaOH, 20.0 cm3 of 0.100 M NaOH is required to reach the end point. Show this calculation.
Answer
moles acid=0.0250×0.0800=2.00×10−3; moles base added=0.0200×0.100=2.00×10−3; end point reached.
11
Question
What is the pH of the solution after adding 25.0 cm3 of 0.0800 M C2H5COOH with 20.0 cm3 of 0.100 M NaOH? (end point)
Answer
pH at end point ~ 8.72.
12
Question
On the pH curve axis, what general shape is expected for a weak acid-strong base titration?
Answer
A buffer region flanked by a sharp rise at equivalence.
13
Question
Which indicator from Table 20.2 is most suitable for titration of CH3COOH with NaOH to reach pH around 4–9?
Answer
Bromophenol blue (pH range 3.0–4.6) is suitable around pH 4.
14
Question
If you start with 25.0 cm3 of 0.0800 M HCN and add 45.0 cm3 of 0.100 M NaOH, what similarity would the pH curves share with C2H5COOH?
Answer
Both show buffering region near pH ≈ pKa and a sharp rise at equivalence.
15
Question
What is the pKa of carbonic acid, H2CO3, given in the text?
Answer
pKa = 6.38.
16
Question
Explain, in terms of equilibrium, how carbonic acid / hydrogencarbonate buffer maintains blood pH at 7.40.
Answer
Equilibrium shifts to oppose pH change by consuming/add CO2, stabilizing HCO3−/H2CO3 ratio.
17
Question
Calculate the [HCO3−]/[H2CO3] ratio in healthy blood given pKa 6.38 and target pH 7.40.
Answer
Ratio = 10^(7.40−6.38) = 10^1.02 ≈ 10.5.
18
Question
What are the conjugate acid-base pairs for the reaction of methanoic acid HCOOH with water? (Label A1,B1 and A2,B2)
Answer
A1: HCOOH; B1: H3O+ ; A2: HCOO−; B2: H2O.
19
Question
If a buffer contains equal amounts of CH3COOH and CH3COONa, what is the pH relative to pKa (CH3COOH 1.78×10−5)?
Answer
pH = pKa = 4.75.
20
Question
What does Ka measure for a weak acid?
Answer
The acid’s tendency to dissociate in water.
21
Question
Why might the measured pH differ from calculated for a weak acid like HIO3 in a real experiment?
Answer
Assumes complete dissociation and ignores activity/concentration changes.
22
Question
In a buffer system, what happens if more CH3COONa is added?
Answer
pH increases; ratio [A−]/[HA] increases.
23
Question
What does the term 'end point' refer to in acid-base titration?
Answer
Point where moles of acid equal moles of base.
24
Question
What is the consequence of a very large Ka on pH?
Answer
Stronger acid, lower pH.
25
Question
Define a buffer solution.
Answer
A mixture that resists pH change upon small additions of acid/base.
26
Question
For a buffer with weak acid HA and conjugate base A−, which quantity controls pH most?
Answer
Ratio [A−]/[HA].
27
Question
Why is indicator color change essential in titration?
Answer
Signals approach to equivalence based on pH range.
28
Question
What is the effect of dilution on pH of a buffer?
Answer
Moderate; pH changes little due to Henderson–Hasselbalch buffering.
29
Question
What is the unit of Ka?
Answer
mol dm−3.
30
Question
What is conjugate acid of CH3COO−?
Answer
CH3COOH.