Problem Based Learning


Answer ALL questions concisely and by given deadline (see Unit Guide). For calculations, you will only get full marks for correct answers and only if you show how you did them.

1: What is the pH of a 0.1 mol/L solution of a weak acid (pKa =3.8)?

2: Match the following diagnoses with the blood gas results;

(a) a baby with pyloric stenosis

(b) a 70-year old with chronic obstructive airways disease

(c) a patient with diabetic ketoacidosis

(d) a laboratory transcription error

1 2 3 4

H+ (nmol/L) 27 50 55 38

PCO2 (kPa) 4.0 11.6 3.9 9.9

PO2 (kPa) 9.1 4.8 12.1 10

HCO3- (mmol/L) 29.7 33.4 14.7 22

3: What is the pH of an acetate buffer prepared from 100 mL of 0.1 mol/L acetic acid solution and 75 mL of a 0.2 mol/L sodium acetate solution? The pKa of acetic acid is 4.76.

4: 79.5 mL of 0.1M citric acid and 22.55 mL of 0.2M Na2HPO4 give a buffer of pH 3.0. What weights of solids would be needed to make up to 200 mL of a buffer pH 3.0 and 50 mmol/L in phosphate. Citric acid comes as the monohydrate (M.Wt = 210), whilst Na2 HPO4 is supplied as the dihydrate (M.Wt = 178).

5: Psychogenic vomiting in a young adult caused dehydration and gave the following biochemical results:

Plasma: Na+ 140 mmol/L

K+ 2.4 mmol/L

Urea 15.5 mmol/L

HCO3- 39 mmol/L

Briefly explain the above results.

6: You are required to prepare a working glucose standard of 8.33 mmol/L from a commercial stock solution of 9.01 g/L in saturated benzoic acid (5 g/L). What volume of glucose solution should be made up to 1L with distilled water? What would be the concentration in g/L of the benzoic acid in the final solution (M.Wt of glucose = 180).

7: Concentrated sulphuric acid is 96% by weight H2SO4 and has a specific gravity of 1.84. Calculate the volume of concentrated acid required to make 750 mL of 1.0 M H2SO4.

8: A patient was admitted to hospital with a crushed chest. On admission her arterial blood gases were:

H+ 63 nmol/L

HCO3- 29 mmol/L

PCO2 10.1 kPa

PO2 6.4 kPa

What do these results indicate?

9: A 56-year old woman was admitted to hospital and was seriously ill and confused. She has systemic oedema and was being treated with frusemide. On admission, the following biochemical results were obtained:

Plasma: Na+ 135 mmol/L Arterial blood: H+ 33 nmol/L

K+ 2.6 mol/L PCO2 9.3 kPa

Cl- 59 mmol/L PO2 12 kPa

Urea 6.8 mmol/L HCO3- 53 mmol/L

(a) What is the evidence that this patient has a mixed acid-base disorder? Identify the key components.

(b) Explain the aetiology of the present blood gas and electrolyte results.

(c) How should the patient be treated.

10: (a) Calculate the pH of a solution of 0.001 mol/L HCl. Would a solution of a weak acid (such as ethanoic) of identical concentration have the same pH? Briefly explain, commenting on any additional information that might be required for the second calculation.

(b) How much greater is the [H+] in a solution of pH 2 than in a solution of pH 7?

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