Polyvalance and Normality
High-Yield Summary
- Polyvalence: an acid or base's capacity to donate/accept more than one H⁺ or OH⁻ per molecule.
- Equivalent: for an acid, the amount donating 1 mole H⁺; for a base, the amount accepting 1 mole H⁺ or donating 1 mole OH⁻.
- Polyprotic acid donates >1 H⁺ per molecule — diprotic H₂SO₄ (2 H⁺), triprotic H₃PO₄ (3 H⁺). Polyvalent base accepts/donates >1 H⁺/OH⁻ — e.g., Ca(OH)₂ (2 OH⁻).
- Normality (N) = equivalents solute / L solution = M × n, where n = equivalents per mole (valence factor).
- Normality directly tracks reactive capacity, making it especially useful for titrations and quantitative acid-base analysis.
Key Terms
- Polyvalence
- An acid or base's capacity to donate or accept more than one H⁺ or OH⁻ per molecule.
- Equivalent
- For an acid: amount donating 1 mole H⁺. For a base: amount accepting 1 mole H⁺, or donating 1 mole OH⁻.
- Polyprotic acid
- An acid capable of donating more than one proton per molecule (e.g., diprotic, triprotic).
- Polyvalent base
- A base capable of accepting more than one proton, or donating more than one OH⁻, per molecule.
- Valence factor (n)
- Number of equivalents each mole of solute supplies — used in N = M × n.
Normality
N = Equivalents of Solute / Volume of Solution (L) = M × n
- n = equivalents per mole — 1 monoprotic/monovalent, 2 diprotic, 3 triprotic, etc.
- Worked example 1 (diprotic): 0.250 M H₂SO₄, n=2 → N=0.500 N.
- Worked example 2 (triprotic): 0.100 M H₃PO₄, n=3 → N=0.300 N.
- Worked example 3 (polyvalent base): 0.150 M Ca(OH)₂, n=2 → N=0.300 N.
Polyprotic/Polyvalent Examples
| Species | Classification (equivalents/mol) |
|---|---|
| H₂SO₄ | Diprotic acid — 2 H⁺ per molecule (n=2) |
| H₃PO₄ | Triprotic acid — 3 H⁺ per molecule (n=3) |
| Ca(OH)₂ | Polyvalent base — 2 OH⁻ per formula unit (n=2) |
Must-Know Points
- A monoprotic acid or monovalent base has exactly 1 equivalent per mole — n=1, so N=M in that case.
- n is the SAME multiplier whether you're describing a polyprotic acid or a polyvalent base — it always equals equivalents supplied per mole.
- Normality was first introduced back in concentration units (N=M×n) — this subtopic extends it specifically to acid-base reactive capacity.
Common MCAT Trap
- Don't confuse 'polyprotic' with automatically meaning 'strong' — H₃PO₄ is triprotic but a WEAK acid; polyvalence describes how many protons it CAN donate, not how completely it dissociates.
- Each successive dissociation step of a polyprotic acid has a different (decreasing) Ka — don't treat all 3 protons of H₃PO₄ as equally easy to remove.
- N = M × n, not M / n — a diprotic acid's normality is DOUBLE its molarity, not half.
Quick Recall
What is the normality of a 0.150 M Ca(OH)₂ solution?
How many equivalents are in one mole of H₃PO₄?
For a monoprotic acid, how does N compare to M?