Chapter 13 · Solutions
13.5Specifying Solution Concentration: Mass Percent
10 min · two checks
Predict
What is the mass percent of a solution made from 5.0 g of salt and 95.0 g of water?
The idea
- Calculate mass percent.
- Use it as a conversion factor.
Mass percent is the solute’s mass divided by the whole solution’s mass, times 100%. The denominator includes the solute. Forgetting and dividing by solvent mass alone is the standard mistake, and it is small only when the solution is dilute. A 5% solution means 5 g solute per 100 g solution, which is a conversion factor: 5 g solute / 100 g solution.
You can solve for any one of solute mass, solution mass, or percent if the other two are known. Keep grams with grams. If volumes are given, you need densities before you pretend volume percent and mass percent are the same. For dilute aqueous solutions they are close; for concentrated ones they are not.
Keep these
- mass % = (g solute / g solution) × 100%.
- Solution mass = solute + solvent.
- Percent is a conversion factor between solute and solution.
Worked path
How many grams of NaCl are in 250 g of a 0.90% by mass solution (physiological saline, approximately)?
Observe
0.90 g NaCl per 100 g solution.