Calculation update from user

Need 0.9g in 100mL and I have got a solution of 4.5g in 100mL.
Half of the 4.5% solution would be 2.25g in 50mL and half again would be 1.125g in 25mL.
If I were to put 25mL of the 4.5% solution into the 100ml WFI I would have 1.125g in 125mL (thinking back - I need 0.9g in 100mL) this is near enough what I want.

I did check with a calculator 1.125g/125mL = 0.9g/100mL

Could you add another 100-150mL to an expandable 100mL bag of WFI so they would seem poor choices.  Therefore there is really only a couple of sensible answers and that would reduced it from a 1:5 guess to a 50:50 choice.  So, even if you can't do the maths, you can still have a sensible guess.

Pharmacy Formulation Calculation Question

What is the percentage of Betamethasone Valerate in the following ointment? Liquid Paraffin 50g Emulsifying was 50mg Betamethasone Valerate 400mg Yellow soft paraffin to 200g.

1. 4%
2. 2%
3. 0.2%
4. 0.4%
5. 0.04%

Students working...?

1- 0.1g/100g
    0.4g     x                      = 0.4 x 0.1 / 100 = 400/200 = 2%

Where did you get 0.1g in 100g? It looks like you are using some sort of formula. You need to visualize what this requires. Writing it down often helps. When I attempted this I wrote down all of the ingredients (but you don't need to in the end as it is made up to 200g).

• Step 1: You need to figure how much betamethasone is in the total amount of the mix.The total weight is 200g as at the end it says it is made up to 200g and the amount in this is 400mg.
• Step 2: Convert to similar units ie 400mg is equivalent to 0.4g.
• Step 3: Create a percentage by now you know 0.4g/200g is what you need. This is quiet easy in your head just half it, giving 0.2g/100g which is your answer 0.2% which is 3.

*"Yellow soft paraffin to 200g" - this is a little ambiguously worded, however it remains as the questions in the exam will not be perfect.

Pharmacy calculation question

A patient requires an intravenous infusion of 0.9%w/v sodium chloride. In your hospital pharmacy department you have Water for Injections, BP and 4.5%w/v sodium chloride solution, BP. Assuming no volume displacements effects, which of the following volumes of 4.5%w/v sodium chloride solution, BP need to be added aseptically to an expandable PVC infusion bag containing 100ml Water for Injections,BP to produce the requisite sodium chloride concentration?

A. 25mL
B. 50mL
C. 100mL
D. 125mL
E. 150mL


To start out this looks pretty tricky! But actually quiet simple. You have a 4.5% solution ie there are 4.5g of Sodium Chloride in 100ml of the solution. 


C1V1=C2V2
4.5xV1=0.9x100 (you do not know the resulting volume of solution ie V2 but assume 100ml for the moment).
So, 4.5xV1=90
V1=90/4.5 = 20ml (you know this ACTUAL answer would be a little larger than this as there is an increase in volume)


So then check this answer with the options available. The closest here is 25ml. 25ml of a 4.5ml solution contains 1.125g of sodium chloride. So 1.125 in a 125ml solution is a 1.125/125x100 which is 0.9. So option A, would be the correct answer.

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