A child weighing 65 pounds required a dose of 5 mg/kg of azithromycin. Given that a 5 ml dose is to be given, what volume of water must be added when the powder is reconstituted? Instruction on the label indicates that dilution to 22.5ml (be adding 12 mL) gives 200 mg azithromycin per 5 mL. (4 marks)

You need to know: 1 lb = 0.45 kg (just under half a kg per lb)

65 lbs * 0.45 = 29.25 kg

The required dose: 29.25 * 5 = 146.25 mg

The question wants you to dilute the 200 mg/5ml solution to 146.25 mg in 5 mls.

Concentration = mass/volume

How much azithromycin is in the 22.5 ml 200mg/5ml Zithromax:

22.5/5 * 200 = 900 mg.

What volume is required to get a 146.25mg/5ml solution:

146.25 mg/5ml = 29.25 mg/ml

= mass/concentration = 900/29.25 = 30.77 ml

The reconstituted solution is 22.5 ml. (It was 12 ml to reconstitute and not 22.5 ml water because of the displacement factor.)

30.77 - 22.5 = 8.27 ml

Add 8.27 ml of water to the reconstituted 22.5ml solution to get a final concentration of 146.25 mg/5ml. (It is ok if you want to round to 1 decimal place)