Physical Science / Unit 6: Solutions, Acids and Bases
6.3 Concentration
Notes
Concentration tells how much solute is in a given amount of solution.
- Mass percent $\displaystyle = \frac{\text{mass of solute}}{\text{mass of solution}} \times 100\%$.
- Example: 20 g of sugar in 180 g of water makes 200 g of solution, so $\displaystyle \frac{20}{200} \times 100\% = 10\%$.
- Molarity $\displaystyle M = \frac{\text{moles of solute}}{\text{liters of solution}}$. 0.5 mol in 0.25 L is $\displaystyle \frac{0.5}{0.25} = 2$ M.
- Adding more solvent makes a solution more dilute; adding more solute makes it more concentrated.
Practice
10 g of salt is dissolved in 90 g of water. What is the mass percent of salt?
Solution mass is $10 + 90 = 100$ g, so $\displaystyle \frac{10}{100} = 10\%$.
A solution has 3 mol of solute in 1.5 L of solution. What is its molarity?
$\displaystyle \frac{3}{1.5} = 2$ M.
What happens to the concentration of a solution when more water is added?
The same amount of solute is spread through more solution.
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