The equivalent molar mass is calculated by dividing the molar mass of a substance by the number of equivalents (n). This 'n' represents the number of positive or negative electrical charges resulting from the substance's reaction. In simpler terms, it's the molar mass adjusted for the substance's reactive capacity.
Understanding the Calculation
- Molar Mass: This is the mass of one mole of a substance, measured in grams per mole (g/mol). It's numerically equivalent to the molecular weight, but expressed in a different unit.
- Number of Equivalents (n): This depends on the specific chemical reaction and the substance involved. For example:
- For acids, n is the number of acidic hydrogen ions (H⁺) that can be released.
- For bases, n is the number of hydroxide ions (OH⁻) that can be released.
- For salts, n is the number of charges on the cation or anion.
- Formula: Equivalent molar mass = Molar mass / n
Examples
- Example 1: A monoprotic acid (like HCl) releases one H⁺ ion. If the molar mass of HCl is 36.5 g/mol, its equivalent molar mass is 36.5 g/mol (36.5 g/mol / 1).
- Example 2: A diprotic acid (like H₂SO₄) releases two H⁺ ions. If the molar mass of H₂SO₄ is 98 g/mol, its equivalent molar mass is 49 g/mol (98 g/mol / 2).
- Example 3: For a gas mixture, as seen in one of the provided examples, the equivalent molar mass is calculated as the weighted average of the molar masses of the individual components of the mixture. For example, a mixture of 5 moles of helium (molar mass 4 g/mol) and 2 moles of hydrogen (molar mass 2 g/mol): Equivalent molar mass = (5 4 + 2 2) / (5 + 2) = 24/7 g/mol ≈ 3.43 g/mol.
Key Differences from Molar Mass
While numerically similar in some cases, the equivalent molar mass differs from the molar mass because it accounts for the substance's reactivity in a specific chemical reaction. The molar mass is an inherent property of the substance, whereas the equivalent molar mass is context-dependent.
Practical Applications
Equivalent molar mass is crucial in various applications, including:
- Titration Calculations: Determining the concentration of solutions during titrations.
- Electrochemistry: Understanding electrochemical reactions and calculating cell potentials.
- Stoichiometry: Carrying out stoichiometric calculations involving reactions.
