# Percent By Weight Formula

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

The percent 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 percent 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 percent weight formula.

25.0g NaCl100mL of solution = X%

Therefore, X = 25%