GPhC Pharmaceutical Calculations

Question 1

A prescription requires 150mL of a 1 in 10,000 solution of potassium permanganate. How many milligrams of potassium permanganate are needed?

Accepted Answer

15mg

Answer

A '1 in 10,000' solution means 1g in 10,000mL. For 150mL: (1g ÷ 10,000mL) × 150mL = 0.015g = 15mg. Remember that 'x in y' concentrations express grams per millilitres.

Question 2

A child weighing 22kg requires amoxicillin at a dose of 25mg/kg/day in three divided doses. The suspension available is 250mg/5mL. What volume should be given per dose?

Accepted Answer

7.3mL

Answer

Total daily dose = 25mg/kg × 22kg = 550mg/day. Per dose = 550mg ÷ 3 = 183.3mg. Volume = (183.3mg ÷ 250mg) × 5mL = 3.67mL, rounded to 3.7mL per dose.

Question 3

How many millimoles of sodium are in 500mL of sodium chloride 0.9% infusion? (Molecular weight of NaCl = 58.5, Na = 23, Cl = 35.5)

Accepted Answer

77 mmol

Answer

NaCl 0.9% = 0.9g per 100mL = 4.5g per 500mL. Moles of NaCl = 4.5g ÷ 58.5g/mol = 0.0769 mol = 76.9 mmol. Since each NaCl molecule gives one Na⁺ ion, sodium content = 77 mmol. (Note: 154 mmol would be per litre.)

Question 4

A pharmacist needs to dilute a 95% v/v ethanol solution to produce 500mL of 70% v/v ethanol. What volume of the 95% solution is required?

Accepted Answer

368mL

Answer

Using C₁V₁ = C₂V₂: 95% × V₁ = 70% × 500mL. V₁ = (70 × 500) ÷ 95 = 35,000 ÷ 95 = 368.4mL. Therefore approximately 368mL of the 95% solution is needed, made up to 500mL with purified water.

Question 5

A patient is prescribed an IV infusion of 1 litre of 5% glucose to run over 8 hours using a giving set that delivers 20 drops/mL. What is the required drip rate in drops per minute?

Accepted Answer

42 drops/min

Answer

Total volume = 1000mL. Total drops = 1000mL × 20 drops/mL = 20,000 drops. Time = 8 hours × 60 min = 480 minutes. Drip rate = 20,000 ÷ 480 = 41.7, rounded to 42 drops per minute.

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GPhC Pharmaceutical Calculations