Solution Concentration and Molarity in General Chemistry

Solution Concentration

Definitions and Types of Solutions

In chemistry, a solution is a homogeneous mixture composed of two or more substances. The solute is the minority component that is dissolved, while the solvent is the majority component that does the dissolving.

• Solute: The substance being dissolved (e.g., sugar in sugar water).

• Solvent: The substance doing the dissolving (e.g., water in sugar water).

Solutions can be described based on the amount of solute relative to the solvent:

• Dilute Solution: Contains a small amount of solute relative to the solvent.

• Concentrated Solution: Contains a large amount of solute relative to the solvent.

Molarity

Definition and Calculation

Molarity (symbol: M) is a common way to express the concentration of a solution. It is defined as the number of moles of solute per liter of solution.

• Formula for Molarity:

$\text{Molarity (M)} = \frac{\text{amount of solute (mol)}}{\text{volume of solution (L)}}$

• Unit: Moles per liter (mol/L), often written as "M" (e.g., 0.2 M NaCl solution).

Example Calculation

If we dissolve 5.8 g of NaCl in enough water to make 500 mL of solution, what is the concentration of the final solution?

• Molar mass of NaCl = 58 g/mol

• Amount of NaCl: $\frac{5.8\,\text{g}}{58\,\text{g/mol}} = 0.1\,\text{mol}$

• Volume of solution: $500\,\text{mL} = 0.500\,\text{L}$

• Molarity: $\frac{0.1\,\text{mol}}{0.500\,\text{L}} = 0.2\,\text{M}$

Example: Dissolving 5.8 g NaCl in 500 mL water yields a 0.2 M NaCl solution.

Solution Dilution

Concept and Formula

In laboratory settings, solutions are often prepared from more concentrated "stock" solutions by dilution. Dilution involves adding solvent to decrease the concentration of solute.

• Dilution Formula:

$M_1V_1 = M_2V_2$

• $M_1$ = initial (stock) molarity

• $V_1$ = volume of stock solution used

• $M_2$ = final (diluted) molarity

• $V_2$ = final total volume after dilution

Example: To prepare 2.00 L of 0.50 M solution from a 10 M stock solution, use:

$(10\,\text{M})V_1 = (0.5\,\text{M})(2.00\,\text{L})$ $V_1 = \frac{(0.5)(2.00)}{10} = 0.1\,\text{L} = 100\,\text{mL}$

So, 100 mL of stock solution is diluted to a total volume of 2.00 L.

Key Terms

• Solute: The substance dissolved in a solution.

• Solvent: The substance in which the solute is dissolved.

• Molarity (M): Moles of solute per liter of solution.

• Dilution: The process of reducing the concentration of a solute in solution, usually by adding more solvent.