How is the equivalent weight of a substance calculated?

How is the equivalent weight of a substance calculated?

• A. As the formula weight
• B. Formula weight divided by the number of reactive units
• C. It is the molar mass
• D. It is the density

Answer: (B)

Equivalent weight reflects how much mass of a substance reacts per unit of reactive capacity. It is found by taking the molar mass (formula weight) and dividing it by the number of reactive units the substance can donate or accept in the reaction. Those reactive units might be the number of H+ ions an acid can furnish, the number of OH− ions a base provides, or the number of electrons exchanged in a redox process. For example, sulfuric acid has a molar mass of about 98.1 g/mol and can furnish two protons per molecule, so its equivalent weight is 98.1 / 2 ≈ 49.0 g/eq. Sodium hydroxide has a molar mass of 40.0 g/mol and provides one OH−, so its equivalent weight is 40.0 g/eq. If a substance donates two electrons in a redox reaction, its equivalent weight would be its molar mass divided by 2. Density is not used in this calculation, and the molar mass alone is not the equivalent weight unless there is only one reactive unit per formula unit.

Equivalent weight reflects how much mass of a substance reacts per unit of reactive capacity. It is found by taking the molar mass (formula weight) and dividing it by the number of reactive units the substance can donate or accept in the reaction. Those reactive units might be the number of H+ ions an acid can furnish, the number of OH− ions a base provides, or the number of electrons exchanged in a redox process.

For example, sulfuric acid has a molar mass of about 98.1 g/mol and can furnish two protons per molecule, so its equivalent weight is 98.1 / 2 ≈ 49.0 g/eq. Sodium hydroxide has a molar mass of 40.0 g/mol and provides one OH−, so its equivalent weight is 40.0 g/eq. If a substance donates two electrons in a redox reaction, its equivalent weight would be its molar mass divided by 2.

Density is not used in this calculation, and the molar mass alone is not the equivalent weight unless there is only one reactive unit per formula unit.