# What is the weight per volume method to calculate concentration?

Jan 1, 2015

A weight/volume percent concentration ($\text{w/v%}$) is defined as mass of solute divided by volume of solution and multiplied by 100%. The mathematical expression for a $\text{w/v%}$ percent concentration is

"w/v%" = ("mass of solute")/("volume of solution") * 100%

As an example, a 5% $\text{w/v}$ $N a C l$ solution would have 5 g of $N a C l$ for every 100 mL of solution. A 25% $\text{w/v}$ $N a C l$ solution would have 25 g of $N a C l$ for every 100 mL of solution, and so on.

Weight/volume percent concentrations are characteristic when solids are dissolved in liquids and are often used because volumes are easier to measure than weights.

Another important reason for using such percent concentrations is the fact that dilute solutions have a density that's generally close to 1 g/mL, which makes the volume of the solution expressed in mL nearly numerically equal to the mass of the solution expressed in grams.

As a conclusion, in order to calculate a $\text{w/v}$ percent concentration, you must determine how much solute you have - its mass in grams - and how much volume of solution you have - in mL.