To prepare 100 mL of acetate buffer at pH 5.06, a technician mixes 0.1 M acetic acid (pKa 4.76) with 0.1 M sodium acetate. Which volumes are correct?

66.7 mL sodium acetate + 33.3 mL acetic acid

50.0 mL sodium acetate + 50.0 mL acetic acid

57.4 mL sodium acetate + 42.6 mL acetic acid

33.3 mL sodium acetate + 66.7 mL acetic acid

Pure water at 37 °C has a pH of about 6.8. This water is:

Neutral, as [H+] still equals [OH-]

Slightly alkaline, as Kw falls on warming

Contaminated with dissolved carbon dioxide

Slightly acidic, as its pH is below 7

A pH meter is calibrated with two standard buffers, pH 7.00 and pH 4.01, before use. The two-point calibration sets:

Both the zero point and the electrode slope

Only the temperature of the solution

The chloride content of the reference electrode

Only the liquid junction potential

A 6-year-old boy with nephrotic syndrome has serum albumin 1.6 g/dL (reference 3.5-5.0 g/dL) and swelling of the face and legs. Which property of his plasma has fallen enough to cause the oedema?

Colloid osmotic (oncotic) pressure

pH, buffers and physical chemistry

13 Kerala MSc MLT Entrance practice questions on pH, buffers and physical chemistry, part of Biochemistry. The first 13 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
To prepare 100 mL of acetate buffer at pH 5.06, a technician mixes 0.1 M acetic acid (pKa 4.76) with 0.1 M sodium acetate. Which volumes are correct?
1. A50.0 mL sodium acetate + 50.0 mL acetic acid
2. B66.7 mL sodium acetate + 33.3 mL acetic acid
3. C57.4 mL sodium acetate + 42.6 mL acetic acid
4. D33.3 mL sodium acetate + 66.7 mL acetic acid
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Answer: B · 66.7 mL sodium acetate + 33.3 mL acetic acid
By Henderson-Hasselbalch, 5.06 = 4.76 + log ([acetate]/[acetic acid]), so log ratio = 0.30 and the ratio is 2:1. Both solutions are 0.1 M, so the volumes are in the same ratio: two-thirds sodium acetate and one-third acetic acid. That is 66.7 mL and 33.3 mL. A pH above the pKa always needs more salt than acid.
- A: Equal volumes give a pH equal to the pKa, 4.76.
- C: 57.4 : 42.6 uses e^0.30 (natural log) instead of 10^0.30.
- D: Inverting the ratio gives a pH of 4.46, below the pKa.
Question 2 · Easy
The relative viscosity of a liquid such as serum is commonly measured with:
1. AA freezing-point osmometer
2. BA stalagmometer
3. CAn Ostwald viscometer
4. DAn Abbe refractometer
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Answer: C · An Ostwald viscometer
The Ostwald viscometer times the flow of a fixed volume of liquid through a capillary tube and compares it with water at the same temperature. Viscosity rises with large proteins, so serum viscosity is raised in conditions such as Waldenstrom macroglobulinaemia and myeloma. Temperature must be controlled because viscosity falls as liquids warm. The stalagmometer, by contrast, counts drops to measure surface tension.
- A: An osmometer measures osmolality from the freezing point.
- B: A stalagmometer measures surface tension by counting drops.
- D: A refractometer measures refractive index, used for urine specific gravity and total protein.
Question 3 · Medium
Pure water at 37 °C has a pH of about 6.8. This water is:
1. ASlightly alkaline, as Kw falls on warming
2. BNeutral, as [H+] still equals [OH-]
3. CContaminated with dissolved carbon dioxide
4. DSlightly acidic, as its pH is below 7
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Answer: B · Neutral, as [H+] still equals [OH-]
The ionisation of water increases with temperature, so Kw at 37 °C is larger than 1 x 10^-14. Both [H+] and [OH-] rise equally, and pH falls to about 6.8 while the water stays neutral. A pH of 7.0 marks neutrality only at 25 °C. For the same reason, pH meters need temperature compensation.
- A: Kw rises, not falls, when water is warmed.
- C: Pure water is stated; the lower pH comes from greater ionisation, not CO2.
- D: Neutrality means equal [H+] and [OH-], not a pH of exactly 7.
Question 4 · Medium
A pH meter is calibrated with two standard buffers, pH 7.00 and pH 4.01, before use. The two-point calibration sets:
1. AOnly the temperature of the solution
2. BBoth the zero point and the electrode slope
3. CThe chloride content of the reference electrode
4. DOnly the liquid junction potential
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Answer: B · Both the zero point and the electrode slope
The glass electrode gives about 59 mV per pH unit at 25 °C, but its offset and slope drift with age and coating. The first buffer (near pH 7) sets the zero point, and the second sets the slope, so that readings between and near them are accurate. The buffers should bracket the expected sample pH. Temperature compensation is set separately.
- A: Temperature is measured or entered separately for compensation.
- C: The reference electrode filling solution is fixed and not adjusted by calibration.
- D: Junction potential is part of the offset but is not the only thing set.
Question 5 · Medium
A 6-year-old boy with nephrotic syndrome has serum albumin 1.6 g/dL (reference 3.5-5.0 g/dL) and swelling of the face and legs. Which property of his plasma has fallen enough to cause the oedema?
1. AViscosity
2. BTotal crystalloid osmolality
3. CColloid osmotic (oncotic) pressure
4. DSurface tension
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Answer: C · Colloid osmotic (oncotic) pressure
Proteins, mainly albumin, cannot cross the capillary wall, so they create the colloid osmotic pressure that holds water in the vessels. When albumin falls, this pressure falls and fluid moves into the tissues. Crystalloids such as sodium and glucose make up most of total osmolality, but they cross capillaries freely and do not hold water in the plasma. The serum osmolality is therefore usually normal in nephrotic syndrome.
- A: Viscosity depends mainly on cells and large proteins but does not move fluid into tissues.
- B: Crystalloids cross the capillary wall freely and do not govern fluid exchange there.
- D: Surface tension plays no part in capillary fluid exchange.
Question 6 · Medium
Two phosphate buffers both have a pH of 6.8: one is 0.1 M and the other 0.01 M. Compared with the 0.01 M buffer, the 0.1 M buffer:
1. AHas a pKa about one unit lower
2. BResists added acid about ten times better
3. CResists added acid less well
4. DHas ten times the hydrogen ion concentration
Answer and explanation in LearnFree account
Question 7 · Hard
A patient's serum gives sodium 140 mmol/L, glucose 180 mg/dL and urea 60 mg/dL. Using the formula 2[Na+] + glucose/18 + urea/6, what is the calculated serum osmolality?
1. A300 mOsm/kg
2. B280 mOsm/kg
3. C160 mOsm/kg
4. D311 mOsm/kg
Answer and explanation in LearnFree account
Question 8 · Medium
In Hay's test, fine sulphur powder sprinkled on urine sinks when bile salts are present. Which physical property of the urine is changed by the bile salts?
1. AIts osmotic pressure is raised
2. BIts viscosity is increased
3. CIts surface tension is lowered
4. DIts specific gravity is lowered
Answer and explanation in LearnFree account
Question 9 · Easy
At a normal arterial pH of 7.40, the ratio of bicarbonate to dissolved carbonic acid (HCO3- : H2CO3) in plasma is about:
1. A1 : 20
2. B10 : 1
3. C20 : 1
4. D1 : 1
Answer and explanation in LearnFree account
Question 10 · Hard
A combination glass pH electrode was left dry in its box over a long holiday. On Monday it responds slowly, the reading keeps drifting, and the two-point calibration with pH 7.00 and 4.01 buffers shows a slope of only 82%. Fresh buffers give the same result. What should the technician do first?
1. ASoak the electrode in KCl storage solution, then recalibrate
2. BCalibrate again using only the pH 7.00 buffer
3. CStore it in distilled water overnight, then recalibrate
4. DSwitch off the temperature compensation and repeat
Answer and explanation in LearnFree account
Question 11 · Medium
Plasma and interstitial fluid are separated by a capillary wall that holds back plasma proteins but lets small ions pass. Because the proteins carry negative charges, at Donnan equilibrium the plasma side has:
1. AEqual sodium and chloride on both sides
2. BMore of both sodium and chloride ions
3. CFewer sodium ions and more chloride ions
4. DMore sodium ions and fewer chloride ions
Answer and explanation in LearnFree account
Question 12 · Hard
A 58-year-old diabetic man in the casualty ward has arterial pCO2 40 mmHg and plasma bicarbonate 12 mmol/L. Using pKa 6.1 and a CO2 solubility factor of 0.03 mmol/L per mmHg, what is his calculated blood pH?
1. A5.58
2. B7.40
3. C7.70
4. D7.10
Answer and explanation in LearnFree account
Question 13 · Easy
A weak acid and its salt resist a change in pH best when the pH of the buffer is:
1. AEqual to the pKa of the acid
2. BExactly 7.0, whatever the pKa
3. CTwo units below the pKa of the acid
4. DOne unit above the pKa of the acid
Answer and explanation in LearnFree account

pH, buffers and physical chemistry

To prepare 100 mL of acetate buffer at pH 5.06, a technician mixes 0.1 M acetic acid (pKa 4.76) with 0.1 M sodium acetate. Which volumes are correct?

The relative viscosity of a liquid such as serum is commonly measured with:

Pure water at 37 °C has a pH of about 6.8. This water is:

A pH meter is calibrated with two standard buffers, pH 7.00 and pH 4.01, before use. The two-point calibration sets:

A 6-year-old boy with nephrotic syndrome has serum albumin 1.6 g/dL (reference 3.5-5.0 g/dL) and swelling of the face and legs. Which property of his plasma has fallen enough to cause the oedema?

Two phosphate buffers both have a pH of 6.8: one is 0.1 M and the other 0.01 M. Compared with the 0.01 M buffer, the 0.1 M buffer:

A patient's serum gives sodium 140 mmol/L, glucose 180 mg/dL and urea 60 mg/dL. Using the formula 2[Na+] + glucose/18 + urea/6, what is the calculated serum osmolality?

In Hay's test, fine sulphur powder sprinkled on urine sinks when bile salts are present. Which physical property of the urine is changed by the bile salts?

At a normal arterial pH of 7.40, the ratio of bicarbonate to dissolved carbonic acid (HCO3- : H2CO3) in plasma is about:

A combination glass pH electrode was left dry in its box over a long holiday. On Monday it responds slowly, the reading keeps drifting, and the two-point calibration with pH 7.00 and 4.01 buffers shows a slope of only 82%. Fresh buffers give the same result. What should the technician do first?

Plasma and interstitial fluid are separated by a capillary wall that holds back plasma proteins but lets small ions pass. Because the proteins carry negative charges, at Donnan equilibrium the plasma side has:

A 58-year-old diabetic man in the casualty ward has arterial pCO2 40 mmHg and plasma bicarbonate 12 mmol/L. Using pKa 6.1 and a CO2 solubility factor of 0.03 mmol/L per mmHg, what is his calculated blood pH?

A weak acid and its salt resist a change in pH best when the pH of the buffer is: