Ka — Science definition
Acid-ionization equilibrium constant for a specified temperature.
Where it is taught
- Acid Strength, Conjugate Pairs & Ka — Acids, Bases & Titration, Physical Science: Chemistry, Grade 11
In use
For HA + H₂O ⇌ H₃O⁺ + A⁻, Ka=[H₃O⁺][A⁻]/[HA] in the introductory concentration approximation.
Acid Strength, Conjugate Pairs & Ka
Questions using ka
Problem 1
Ka=10⁻⁶. Find pKa.
Hints
Write the relationship first; substitute with consistent units.
Answer and explanation
6
−log 10(10⁻⁶)=6.
Problem 2
Find pKa for Ka=10⁻⁴.
Hints
List the given quantities, select a relationship, and check units before substituting.
Answer and explanation
4
−log 10(10⁻⁴)=4.
Problem 3
For a hypothetical monoprotic acid with C=0.200 M and Ka=2.0×10⁻⁵, estimate hydronium and pH, then test the small-ionization approximation.
Answer and explanation
x≈√(KaC)=√(4.0×10⁻⁶)=0.0020 M. pH≈2.70. x/C≈0.010=1%, supporting the approximation for ordinary classroom accuracy under the stated species model.
x≈√(KaC)=√(4.0×10⁻⁶)=0.0020 M. pH≈2.70. x/C≈0.010=1%, supporting the approximation for ordinary classroom accuracy under the stated species model.
Response criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the chemical mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.
Problem 4
Which acid is stronger in a comparable aqueous model: Ka=10⁻³ or 10⁻⁶?
Answer and explanation
Ka=10⁻³ implies more extensive ionization.
Response criteria
- State a scientifically accurate conclusion.
- Show the calculation or mechanism that supports it.
- Include units, conditions, or limits where relevant.