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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

Example 1

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


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.


Substitute the known values in the percent weight formula.

25.0g NaCl100mL of solution = X%

Therefore, X = 25%