1. Percent by Mass - Direct

1. Place a small volume of a solution into a clean and oven-dried beaker or crystallization dish.
2. After accurately determining the precise total mass of the solution, heat the beaker or dish on a hotplate or in an oven to drive off the water. Slow evaporation is the best method, as boiling can result in splattering of the solution.
3. Once the solvent has evaporated, cool the remaining solid (solute) and determine the mass.
4. Calculate the mass percent as:
   

2. Percent by Mass - Using a Calibration Curve

1. Make a series of standard solutions by dissolving known quantities of solute into a solvent. Five standards are recommended and should range from the minimum to the maximum expected percent compositions.
   1. If the approximate value is unknown, produce a series of solutions ranging from 0% through the solubility maximum of the solute in water. Reference tables are available to indicate the maximum solubility for many solids in water over a variety of temperature ranges.
   2. For example, to produce a 35% (w/w) sodium chloride (NaCl) solution, add 35 g to a flask, then add 65 g (or 65 mL, as volume can also be used when measuring pure water since the density is known to be 1.0 g/mL at ambient temperatures) of water, and mix the solution until the solid is dissolved. Because mass is additive, this would be the same as adding enough water so the total solution has a mass of 100 g.
2. After measuring the mass of a precise volume of each of the standard solutions, calculate the density as:

3. Data Analysis

1. Plot the density values against the percentage by mass of the standard solutions and determine the slope of the line. This slope corresponds to the density vs. mass percent of the solution, fitting a linear equation, y = mx + b (Figure 1).
2. Calculate the density of the unknown sample solution by obtaining the mass of the specific volume of solutions. Now that the slope of the line is known, determine the percent by mass of solute by solving for "x" using the measured density and slope from the calibration curve.