Calcs Flashcards
WHat is key to remember in calcs questions involving ampoules
Ampules cant be reused so one ampoule = 1 dose
What does Molar mean?
Moles per Litre
What is the equation for simple dilution?
C1V1 = C2V2
C1 = Initial concentration
V1= Initial volume
C2 = Final concentration
V2 = Final volume
The diluent volume is the DIFFERENCE between V2 and V1
Soulibility
What does 1 in 400 mean?
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.
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?”
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:
What do the following mean?
5000 ppm (w/w) fluoride toothpaste
225 ppm (w/v) fluoride rinse
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
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.
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.
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.
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
What does 1dL mean ?
1dL = 100ml
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)
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
What is key to remember about dilutions
If you are diluting a 10% solution, then the final concentration MUST be less than 10%.
Look at these displacement examples
Drug
Example of displacement volume in practice
Amoxicillin 250mg/5mL sugar-free powder for reconstitution (100mL)
The directions require 82mL of potable water (diluent) to be added to reconstitute 100mL of 250mg/5mL amoxicillin sugar-free suspension. In this case, 18mL has been displaced
Ceftriaxone powder for infusion fluid
The directions require 3.5mL lidocaine 1% (diluent) added to 1.0g ceftriaxone powder for injection. This produces a final volume of 4.1mL. That is, 1.0g of powder displaces 0.6mL of diluent.
Each piperacillin/tazobactam 4g/0.5g powder for solution for infusion
If you reconstitute a vial of piperacillin/tazobactam 4.0g/0.5g powder for solution for infusion with 20mL 0.9% sodium chloride for injection (diluent), the final volume is 23.15mL. The powder will therefore displace 3.15mL, which means each 1.0g of powder has a displacement volume of 0.7mL (3.15mL divided by 4.5g).
