Here's why:

* w/w stands for "weight by weight".

* It expresses the concentration of a solution as the mass of solute divided by the mass of the solution, multiplied by 100.

* Since the mass of the solution is the sum of the mass of the solute and the mass of the solvent, using 100g of solvent makes the calculation straightforward.

For example, if you dissolve 5g of salt in 100g of water, the mass percentage would be:

(5g salt / (5g salt + 100g water)) * 100 = 4.76% (w/w)