Equivalent mass in redox reactions

In a redox reaction the equivalent mass of a substance is found by dividing its molar mass by the number of electrons one formula unit accepts (oxidising agent) or gives (reducing agent): E = M/n(e⁻). For example K₂Cr₂O₇ accepts 6 e⁻ in acidic medium: E = 294/6 = 49; Na₂SO₃ gives 2 e⁻ when oxidised to SO₄²⁻: E = 126/2 = 63. So E of one substance depends on the reaction conditions: KMnO₄ in acidic medium has E = 158/5 = 31.6, in alkaline medium E = 158/1 = 158. The law of equivalents: substances react in masses proportional to their equivalent masses, that is m₁/E₁ = m₂/E₂ (equal numbers of equivalents). This law lets us find the masses of reducing and oxidising agents without writing the whole equation. Writing the half-reaction to see how many electrons move is the most reliable way. Note: equivalent mass is an older term; current practice does the same calculation with moles and the number of electrons.