% (w/v) explained

I know a lot of you find % (w/v) as a way to express concentrations a little tricky. Here is a % (w/v) question to help me explain....

Question:

Express 4g in 2L as % (w/v)

With % (w/v) you want to find out what percentage the weight you have (in grams) is of the volume you have (in millilitres). It is important to always have your units in grams and ml when dealing with % (w/v).

In this specific question, you need to find out what percentage 4 g is of 2 L of water, which is 2000 ml...

So to figure this out you could do: 4/2000 x 100 = 0.2 % (w/v).

If that doesn't make sense to you, you could do 2000/100 = 20 -> so now you know that 1 % of 2000 is 20. Then you could find out what percentage 4 is of 2000 by doing 4/20 = 0.2 % (w/v).

If you were then making up this solution in the lab, you would weigh out your 4 g of solute, and then put it into a 2L volumetric flask - you would then make the volume up to 2L. You would not take 4 g of solute and add to it 2L of water!!! - then your final volume will be more than 2L, right?

Hope that clears this up...