Lessons · Pharmacy · percent strength, weight in volume
Percent strength: grams in a hundred millilitres
A percent strength written w/v is the number of grams in every 100 mL of the liquid.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
Every bag, bottle and irrigation in the place is labelled this way, and the question that follows is always the same: how much solid is actually in this container? The percent alone does not answer it; the percent and the volume together do.
How to think about it
Count how many hundreds of millilitres the container holds, then multiply the percent by that count. That is the grams. Going the other way, grams divided by millilitres, times 100, is the percent.
Worked example
0.9% w/v = 0.9 g in 100 mLNine tenths of a gram in every hundred millilitres.
0.9 g = 900 mg, so 900 mg / 100 mL = 9 mg/mLThe same strength in the units a dose is written in.
1,000 mL of 0.9% holds 0.9 x 10 = 9 gA litre is ten lots of a hundred millilitres.
300 mL of 5% holds 5 x 3 = 15 gCount the hundreds, then multiply.
Your turn
How many grams are in 250 mL of a 4% w/v solution?
4 x (250 / 100) = g
Solve one, graded on the server
The trap
Reading a percent as if the container size did not matter. 5% is 5 g in 100 mL and 25 g in 500 mL; the percent is a rate, not an amount.