a solution is made up of 20 ml of isopropyl alcohol topped up to 200 ml of water. what is the concentration of this solution?

Answer 1

The concentration will depend on the concentration of the original isopropyl alcohol (IPA) used to make the dilution. Was it pure (100%) IPA? If so, then you would have ...

(20 ml)(100% IPA) = (200 ml)(x %)

x = 10% IPA

Answer 2

There are several ways you can express concentration, one of which is a volume/volume % (v/v%). In this case, you have 20 mls of isopropyl alcohol that you topped off with water to make a combined volume of 200 mls (you added 180 mls to do this). Note, a percent is an expression to represent a part out of a whole, so 20 mls of isopropyl alcohol is a part out of a whole solution of 200 mls. So, you're concentration will be 20 mls ÷ 200 mls of total solution × 100%, which gets you 10% (v/v). Hope this explanation helps.

I also wanted to elaborate on what the other tutor mentioned. If the isopropyl alcohol is 100% isopropyl alcohol, then the concentration would indeed be 10%. However, say you used 20 mls of 70% isopropyl alcohol, then the concentration would be different. Like the other tutor showed, you can use a useful equation, C₁V₁=C₂V₂, to solve for the new concentration with the addition of water. But, in other words, 70% isopropyl alcohol means you have 70mls of pure alcohol in 100 mls of solution. So if you had 20 mls, then you would have 0.7× 20mls or 14 mls of pure isopropyl alcohol. Now say you topped that with 180mls of water to make the new solution 200 mls, then your concentration would be 14mls ÷200mls × 100% or 7% (v/v).