Concentration by Mass
Mass percent (often denoted as %w/w or weight/weight percent) is one of the simplest and most robust ways to express the concentration of a mixture. Because it is based entirely on mass, it is completely immune to temperature changes, unlike volume-based measurements.
Mass percent simply tells you how many grams of solute are present in exactly 100 grams of the total solution.
For instance, a 5% by mass solution of NaCl means that for every $100 , \text{grams}$ of the entire salt-water mixture, exactly $5 , \text{grams}$ of it is pure NaCl, and the remaining $95 , \text{grams}$ is water.
Calculating Mass Percent
To calculate mass percent, you divide the mass of the solute (the substance being dissolved) by the total mass of the entire solution (solute + solvent), and then multiply by 100.
The Formula
Example Calculation
Imagine you dissolve $20 , \text{grams}$ of sugar into $180 , \text{grams}$ of coffee. What is the mass percent of the sugar?
- Calculate Total Solution Mass: $20 , \text{g} , \text{(sugar)} + 180 , \text{g} , \text{(coffee)} = \mathbf{200 , \text{g} , \text{total}}$.
- Divide Solute by Total: $20 / 200 = \mathbf{0.10}$.
- Multiply by 100: $0.10 \cdot 100 = \mathbf{10%}$.
The resulting beverage is 10% sugar by mass.