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Can you help your pre-reg answer these revision calculations?

Part one of the GPhC pre-reg assessment is made up of 40 calculation questions
Part one of the GPhC pre-reg assessment is made up of 40 calculation questions

Are you able to help your pre-reg with these revision calculations?

Your pre-reg has their GPhC registration exam this week, so you decide to give them two calculation questions to complete. After checking their answers, you inform them that unfortunately they have got both questions wrong.

Can you spot their errors and answer these questions correctly?

Question one: potassium permanganate strength

A patient is directed to use 40ml of a 1 in 8,000 solution of potassium permanganate each day for one week. The prescription directs you to make a solution 10 times this strength.  

How much potassium permanganate, in micrograms, will be required to dispense this prescription?

Here is the pre-reg’s working out:

40ml x 7 days (one week) = 280ml of 1 in 8,000

10 times stronger, so must be 1 in 800 

So 28ml of 1 in 800

1 in 800 = 0.00125%

0.00125g in 100ml

So 0.00035g in 28ml

= 0.35 mg

This answer is incorrect. What answer did you get?

Click here for the correct working out and answer

One method of working out the answer:

40ml x 7 days (one week) = 280ml of 1 in 8,000

= 1g in 8,000ml

= 1,000mg in 8,000ml

= 1mg in 8ml

= 35mg in 280ml

= 35,000mcg in 280ml

An alternative method for working out the answer is:

1 in 8,000 = 1g in 8,000ml = 0.0125g in 100ml = 0.0125%w/v

C1 x V1 = C2 x V2 (C1 = 0.125%w/v; V1 = ?; C2 = 0.0125%w/v; and V2 = 280ml)

0.125%w/v x V1 = 0.0125%w/v x 280ml

0.0125%w/v x 28ml = 3.5

3.5/0.125%w/v = 28ml

Therefore 28ml of 0.125%w/v

0.125%w/v = 0.125g in 100ml

= 125mg in 100ml

= 35mg in 28ml

(convert from milligrams to micrograms)

= 35,000mcg in 28ml

Learning points

  • The question is asking for the amount and not the concentration
  • The amount of potassium permanganate does not change when going from 1 in 800 to 1 in 8,000. There is still one gram in both. However, the concentration (%w/v) has changed
  • 1 in 800 is 0.125% and not 0.00125%
  • The question asked for the answer in micrograms and the answer was given in milligrams. It is important to always look at the answer grid and the units for the question before answering.
Question two: hydrocortisone dilution

Mrs B, 57, weighs 74 kilograms and has been prescribed 500g of a 1 in 4 dilution of hydrocortisone butyrate 0.1% ointment. The consultant dermatologist wants the pharmacy to prepare the ointment using emulsifying ointment as the diluent.

How many grams of emulsifying ointment are needed to prepare the requested ointment?

Here is the pre-reg’s working out:

1 in 4 = 5 parts

500g/5 = 100g per part

So, one part emulsifying ointment and four parts hydrocortisone cream

The answer is 100g emulsifying ointment

This answer is incorrect. How would you work out the answer?

Click here for the correct working out and answer

There is one method to work out the answer to this calculation:

1 in 4 = 4 parts

500g/4 = 125g per part

Therefore one part hydrocortisone ointment and three parts emulsifying ointment

3 x 125g = 375g

The answer is 375g emulsifying ointment

Learning points

  • 1 in 4 = 4 parts and not 5 parts
  • It is important not to get the number of each component confused
  • The strength of hydrocortisone butyrate is not important in this calculation, nor is the weight of the patient. Often a question will be worded with information that is not directly relevant to the calculation itself.

Make sure you understand the rounding instructions for GPhC calculations. These can be found on the front of the answer grid. There is also a copy available in the Pre-registration Manual on the GPhC website.

It is very important to have answered the 12 example calculations and understand how to get to the correct answer. Rounding should always occur at the end of the calculation, unless you are working out a dose. Please also look at the learning points from previous sittings for more examples.


Babir Malik is a teacher practitioner at Bradford University and the Green Light Campus masterclass lead

Mr Malik does not set or write questions for the GPhC registration assessment and does not imply that these calculations will come up in the exam. However, both questions relate to the GPhC framework. The author adds that there may be other ways to work out these questions, but it does not matter how you work out a question as long as you get to the correct answer.

Check out C+D’s top tips to prepare for June's GPhC pre-reg exam

You can listen to the podcast in full below. Alternatively, subscribe to all of C+D's podcasts on iTunes by clicking here or by searching 'Chemist+Druggist podcast' on your preferred android podcast app.

GPHC revalidation with chemist and druggist for pharmacy professionals

Where you able to answer the above calculations?

Dave Downham, Manager

RTFQ - Read The Question.

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