# Percent By Weight Formula

Solutions can be described in other concentration besides molarity, normality or molality. Solutions are sometimes represented in terms of relative per cent concentration of solute in a solution. To determine the weight per cent of a solution, divide the mass of solute by mass of the solution (solute and solvent together) and multiply by 100 to obtain per cent.

The per cent by weight formula can be expressed as

$\dpi{120}&space;\large&space;Percent\;&space;by\;&space;weight=\frac{gram\;&space;of\;solute}{&space;100g\;&space;of\;&space;solution}$

Example 1

Calculate how many grams of NaOH are required to make a 30.0% solution by using De-ionized water as the solvent.

Solution

Substitute the numbers that are known in the basic formula for per cent weight

X g of solute in 100g of solution = 30.0%

30.0 g of NaOH = X

Example 2

Determine the % of a solution which has 25.0g of NaCl dissolved in100mL of De-ionized water.

Solution

Substitute the known values in the per cent weight formula.

25.0g NaCl 100mL of solution = X%

Therefore, X = 25%