A child weighing 50 pounds requires a dose of 7 mg/kg of drug X. Given that a 5 mL dose is to be given each time, what volume of water must be added when the drug X powder is reconstituted? Instruction on the label indicates that dilution to 120 mL (by adding 95 mL of water) gives 200 mg drug X per 5 mL. 

(4 marks) 

1kg = 2.2 lbs

Weight of child: 50 lbs

Convert to kg: 50/2.2 = 22.7 kg

Dose if drug X: 7 mg/kg -> 158.9 mg

158.9 mg/ 5 ml requested = 31.8 mg/ml

When reconstituted with 95 ml of water you get 120 ml of 200 mg of X per 5 ml

Total amount of X in the bottle:

120/5 * 200mg = 4800 mg

Volume of 31.8mg/ml solution:

Concentration = mass/volume

4800/31.8 = 150.9 ml

Volume of water to be added to reconstituted solution

150.9 - 120 = 30.9 ml