NAME ___________________________________ SUPPLEMENT: BUFFERS: INTERPRETATING THE
HENDERSON-HASSELBALCH EQUATION
HENDERSON-HASSELBALCH EQUATION:
pH = pKa + log ([conj. base])
[acid]
1) Calculate the pH of a buffer solution at 25 °C, that is a mixture of 0.818 M acetic acid (Ka = 1.8 x10-5) and
0.172 M sodium acetate.
Hint 1: * Note that this is a mixture of a
weak acid, and a salt of that weak
acid…This sets up a common-ion effect
….and that can lead us right to buffers.
-5
*pKa = -log (1.8 x 10 ) = 4.74
*pH = 4.74 + log (0.172)
0.818
Hint 2:* Try Henderson-Hasselbalch
*pH = 4.74 – 0.6772
*pH = 4.06
1) 4.06
2) 5.43
Hint 3:* You’re given the Ka so find the
pKa since H.H.eq. requires the pKa
3) 8.57
4) 8.37
5) 9.92
Hint 4:* Do you grasp that the
concentration of the conjugate base (the
acetate ion) = its molarity …because
sodium acetate dissociates completely in
water …thus 0.172 M gives 0.172 moles
of acetate ion….?
2) The Ka of acetic acid is 1.8 x10-5 at 25°C. Calculate the pH of a buffer prepared by combining 50.0 mL of
1.00 M potassium acetate and 50.0 mL of 1.00 M acetic acid.
1)
2)
3)
4)
5)
1.705
0.851
3.406
4.745
2.383
3) Calculate the pH of a solution prepared by dissolving 0.25 mol of benzoic acid (C7H5O2H) and 0.15 mol of
sodium benzoate (NaC7H5O2) in water sufficient to yield 1.00 L of solution. The Ka of benzoic acid is
6.46 x 10-5
1)
2)
3)
4)
5)
4.41
3.97
2.40
10.08
4.19
4) Calculate the pH of a solution prepared by dissolving 0.15 mol of benzoic acid (HBz) and 0.30 mol of
sodium benzoate in water sufficient to yield 2.00 L of solution. The Ka of benzoic acid is 6.46 x 10-5
1)
2)
3)
4)
5)
2.52
3.89
10.16
4.49
4.20
5) What is the pH of a buffer solution that is 0.211 M in lactic acid and 0.111 M in sodium lactate? The
Ka of lactic acid is 1.37 ×10-4.
1)
2)
3)
4)
5)
14.28
10.43
5.48
4.13
3.58
Answers for 1 – 5: 1) 1
2) 4
3) 2
4) 4
5) 5
6) The addition of hydrofluoric acid and __________ to water produces a buffer solution.
1) HCl
2) NaNO3
3) NaF
4) NaCl
5) NaBr
7) Which one of the following pairs cannot be mixed together to form a buffer solution?
1)
2)
3)
4)
5)
NH3, NH4Cl
KF, HF
NaC2H3O2, HC2H3O2
H3PO4, KH2PO4
RbOH, HBr
8) What does a buffer do?
1)
2)
3)
4)
Helps to resist a change in pH
Helps to prevent any neutralization
Keeps the cation concentration constant
Keeps the anion concentration constant
9) Reflecting upon the Henderson-Hasselbalch equation, what substances are present in a buffer?
1)
2)
3)
4)
Only a salt capable of hydrolyzing water
A weak base or acid, and its salt
Only a weak base or a weak acid
An acid only
10) What change will be caused by addition of a small amount of KOH to a solution containing fluoride ions &
hydrogen fluoride?
1) The concentration of hydronium ions will increase significantly.
2) The concentration of fluoride ions will increase as will the concentration of hydronium ions.
3) The concentration of fluoride ion will decrease and the concentration of hydrogen fluoride will increase.
4) The concentration of hydrogen fluoride will decrease & the concentration of fluoride ions will increase.
5) The fluoride ions will precipitate out of solution as its acid salt
11) Which of the following pairs of compounds could create a suitable buffer system?
1)
2)
3)
4)
5)
ammonia and potassium hydroxide
sodium acetate and acetic acid
carbonic acid and sodium chloride
sodium chloride and sodium hydroxide
sodium sulfate and sodium hydroxide
Answers 6 – 11: 6) 3
7) 5
8) 1
9) 2
10) 3 11)
11) 2