Question

a solution is prepared by dissolving 15 gram of sodium chloride in 200 grams of H2O what is the mass by mass percentage of NaCl in this solution

Answer 1

You’re looking for % w/w, so :

Total weight of solution = weight of solute + weight of solvent : (15 + 200)g = 215g

Weight of solute = 15g

So, %w/w = 15/215 = 6.976% or approx 7% w/w

If you were doing % w/v :

Assuming the addition of salt DOES NOT INCREASE SOLUTION VOLUME :

15g/200ml = 7.5% w/v

But this doesn’t hold true, as adding salt to water will increase the volume, but not by 1ml for 1g (more like 1ml for every 2g)

If you were doing this properly :

15g made up to 200ml with water = 7.5% w/v