Chemistry / Unit 10: Solutions, Acids and Bases
10.2 Molarity and Dilution
Notes
Molarity ($M$) measures concentration: moles of solute per liter of solution.
- $\displaystyle M = \frac{\text{mol solute}}{\text{L solution}}$. Convert mL to L by dividing by 1000.
- Example: 11.69 g NaCl (58.44 g/mol) is 0.2000 mol; in 500.0 mL, $\displaystyle M = \frac{0.2000}{0.5000} = 0.400$ M.
- Moles from a solution: $\text{mol} = M \times V$.
- Dilution adds solvent; moles of solute stay the same: $M_1V_1 = M_2V_2$.
- Example: 25.0 mL of 2.00 M diluted to 100.0 mL gives $\displaystyle M_2 = \frac{(2.00)(25.0)}{100.0} = 0.500$ M.
Practice
What is the molarity of a solution with 0.500 mol of solute in 250 mL of solution?
$\displaystyle \frac{0.500 \text{ mol}}{0.250 \text{ L}} = 2.00$ M. Remember to change 250 mL to 0.250 L.
What mass of NaOH (40.00 g/mol) is needed to make 250.0 mL of 0.100 M NaOH?
$\text{mol} = 0.100 \times 0.2500 = 0.0250$ mol, and $0.0250 \times 40.00 = 1.00$ g.
How much 6.00 M HCl is needed to make 600.0 mL of 0.500 M HCl?
$\displaystyle V_1 = \frac{M_2V_2}{M_1} = \frac{(0.500)(600.0)}{6.00} = 50.0$ mL.
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