Dilutions in Chemistry: Concepts and Calculations

Dilutions

Concept of Dilution

In chemistry, a dilution involves reducing the concentration of a solute in a solution, typically by adding more solvent. This process is fundamental in laboratory settings for preparing solutions of desired concentrations from a more concentrated stock solution.

• Stock Solution: A concentrated solution that will be diluted for laboratory use.

• Dilution: The process of adding more solvent (usually water) to a solution to create a lower concentration.

Example: If each sphere represents a mole of solute, arranging solutions from least to most concentrated depends on the ratio of solute to solvent. More solute per volume means higher concentration.

Dilution Calculations

Dilutions are quantitatively described by the equation:

$M_1 V_1 = M_2 V_2$

• M1: Initial molarity (concentration) of the stock solution

• V1: Volume of the stock solution to use

• M2: Final molarity (concentration) after dilution

• V2: Final total volume after dilution

This formula ensures that the number of moles of solute remains constant before and after dilution.

Worked Example

Problem: What volume (in mL) of 7.5 M H2SO4 must be used to prepare 3.5 L of 2.1 M solution?

• Given: $M_1 = 7.5$ M, $M_2 = 2.1$ M, $V_2 = 3.5$ L

• Find: $V_1$

Using the dilution formula:

$M_1 V_1 = M_2 V_2$

$V_1 = \frac{M_2 V_2}{M_1}$

$V_1 = \frac{2.1 \times 3.5}{7.5} = 0.98$ L $= 980$ mL

Answer: 980 mL of 7.5 M H2SO4 is required.

Key Points

• Always add acid to water, not water to acid, for safety.

• Units for volume must be consistent (convert L to mL if necessary).

• Concentration decreases as more solvent is added, but the amount of solute remains unchanged.