Calcs

Question 1

WHat is key to remember in calcs questions involving ampoules

Answer 1

Ampules cant be reused so one ampoule = 1 dose

Question 2

What does Molar mean?

Answer 2

Moles per Litre

Question 3

What is the equation for simple dilution?

Answer 3

C1V1 = C2V2
C1 = Initial concentration
V1= Initial volume
C2 = Final concentration
V2 = Final volume

The diluent volume is the DIFFERENCE between V2 and V1

Question 4

Soulibility
What does 1 in 400 mean?

Answer 4

You must understand what expressions of solubility mean.

Solubility is normally expressed in parts. Details of the terms of solubility can be found in the British
Pharmacopoeia (BP). Please read the General Notices of the British Pharmacopoeia for more details.

Typical statements of solubility would be: The solubility of Drug Y in water is 1 in 5
-This means that 1 gram of the drug would dissolve in a minimum of 5ml of water. The solubility of Drug X is 1 in 400 in alcohol
-This means that 1 gram of the drug would dissolve in a minimum of 400ml of alcohol.

Question 5

Why is rounding key sometimes mid question rather than at the end?

Example scenario: “A patient needs 5 mg/kg of Drug X, twice daily for 7 days. The child weighs 19.6 kg. The medicine is available as a 50 mg/5 mL oral suspension. What total volume (in mL) of the medicine should be supplied?”

Answer 5

Calculate individual dose:

5mg/kg × 19.6 kg = 98
mgperdose
5mg/kg×19.6kg=98mgperdose
Round this dose to the nearest sensible amount (based on formulation):

The suspension is 50 mg in 5 mL → 10 mg/mL

So 98 mg ÷ 10 = 9.8 mL per dose → round to 10 mL per dose

Total doses:

2 doses/day × 7 days = 14 doses

Total volume required:

10 mL/dose × 14 doses =140mL
10mL/dose×14doses=
140mL
​

💡 Why rounding earlier matters:
If you used 9.8 mL/dose instead and rounded only at the end:

Question 6

What do the following mean?

5000 ppm (w/w) fluoride toothpaste

225 ppm (w/v) fluoride rinse

Answer 6

ppm (w/w)

5000 ppm (w/w) fluoride toothpaste

5,000g fluoride in 1,000,000g of final product weight

ppm(w/v)

225 ppm (w/v) fluoride rinse

225g fluoride in 1,000,000mL of final product volume

Question 7

A 10ml ampoule of sodium bicarbonate solution is diluted 3 parts of drug to 27 parts of infusion fluid. The final concentration of the resulting infusion fluid is 0.84%. What is the initial concentration of sodium bicarbonate in the 10mlL ampoule as a percentage (%)? Give your answer to one decimal place.

Answer 7

Correct method:

3 parts of sodium bicarbonate + 27 parts of infusion fluid = 30 parts of mixture

Dilution factor is 30/3 or 10 times dilution

Initial concentration = 0.84% x 10 = 8.4% to one decimal place.

Question 8

You have been asked to aseptically prepare a 0.9% (w/v) sodium chloride infusion by adding 30% (w/v) of concentrated sodium chloride solution to a 250ml bag of sterile water for injection. How much in millilitres (ml) of 30% concentrated sodium chloride would you add to the bag to produce the desired strength? Give your answer to one decimal place.

Answer 8

You need to correctly interpret the dilution process to avoid errors. Note that the final volume after adding 30% (w/v) concentrated sodium chloride is not 250ml, it is (250ml + volume of sodium chloride added). This is where some candidates get it wrong. The correct calculation is set out below.

C1 = initial concentration of sodium chloride = 30%

C2 = final concentration of sodium chloride = 0.9%

V1 = initial volume of 30% sodium chloride = ?

V2 = final volume of sterile water and 30% sodium chloride or 250 + V1

Use modified C1V1 = C2V2

We know V2 = V1 + 250

30V1 = 0.9(250+V1)

30V1 = 225ml + 0.9V1

29.1V1 = 225ml

V1 = 225ml/29.1

V1 = 7.731958763ml rounded to 7.7ml

Correct answer: 7.7ml

Question 9

What does 1dL mean ?

Answer 9

1dL = 100ml

Question 10

A 60-year-old man has just received a prescription for metronidazole cream 3mcg/1g, to be applied twice daily. Express the concentration in parts per million (ppm)

Answer 10

This is a simple question requiring knowledge of the concept of concentration and conversion of units. First, you should be able to define parts per million that is 1g in 1 000 000g or 1mg in 1 000 000mg or 1mcg/1 000 000mcg. Second, you need to convert 3mcg/1g to have common units in the denominator and numerator. You can either choose a common unit of g, mg or mcg, it does not matter what you choose. Let us work with mcg, then 3mcg/(1g x 1 000mg/1g x 1 000mcg/1mg) = 3mcg/1 000 000mcg = 3ppm

Correct answer: 3ppm