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Percent by Weight Formula
In addition to molarity, normality, or molality, the solution can also be described in other concentrations. A solution may be expressed as a relative percentage concentration of solute in the solution. To determine the weight percent of a solution, divide the mass of the solute by the mass of the solution (the solute plus the solvent) and multiply by 100 to get the percentage.
Formula
Per cent by weight = gram of solute/100 g of solution
Solved Examples
Q1: Calculate the number of grams of NaOH needed to make a 30.0% solution using deionized water as the solvent.
Solution:
Substitute the value of the weight percent known in the basic formula
X g of solute in 100g of solution = 30.00.0g of NaOH = X
Therefore, dissolve 30g of NaOH in 70g of deionized water. A 30% solution of NaOH in water should be produced. In addition, any amount of NaOH can be dissolved in deionized water in a 3:7 ratio to form a 30% NaOH solution.
Q2: Determine the percentage of a solution containing 25.0 g NaCl dissolved in 100 mL deionized water.
Solution:
Given,
Solute mass (NaCl) = 25 g
Solvent mass (deionized water) = 100 g (1 ml deionized water weighs about 1 g)
Therefore, solution mass = solute Mass + mass of solvent = 100 g + 25 g = 125 g
Mass% of NaCl in this solution = (25 g / 125 g) 100% = 20%
Therefore, the mass% of NaCl in solution is 20%.
