AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Ka is the dissociation constant for a weak acid.

    Full specification wording

    Weak acids and bases, Ka, for weak acids (A-level only): Ka is the dissociation constant for a weak acid.

    Ka is the equilibrium constant for the dissociation of a weak acid: for HA ⇌ H⁺ + A⁻, Ka = [H⁺][A⁻]/[HA], with units mol dm⁻³.

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 2 of 2
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Ka is the equilibrium constant for the dissociation of a weak acid: for HA ⇌ H⁺ + A⁻, Ka = [H⁺][A⁻]/[HA], with units mol dm⁻³. Water does not appear, because it is the solvent. To find the pH of a weak acid on its own, two approximations are made: the H⁺ from water is negligible, so [H⁺] = [A⁻]; and so little acid dissociates that [HA] at equilibrium equals the concentration made up. Then Ka = [H⁺]²/[HA], so [H⁺] = √(Ka × [HA]). For butanoic acid, Ka = 1.51 × 10⁻⁵ mol dm⁻³, so a 0.100 mol dm⁻³ solution gives [H⁺] = 1.23 × 10⁻³ mol dm⁻³ and pH = 2.91. The logarithmic form pKa = −log₁₀Ka ranks acids compactly: a larger Ka, and so a smaller pKa, means a stronger acid.

    What earns the marks

    • The expression Ka = [H⁺][A⁻]/[HA].
    • The units mol dm⁻³.
    • Assuming [H⁺] = [A⁻] and that [HA] at equilibrium equals the initial concentration.
    • pKa = −log₁₀Ka and the reverse conversion Ka = 10⁻ᵖᴷᵃ.
    • Identifying the acid with the larger Ka, or the smaller pKa, as the stronger acid.

    Common mistakes and how to put them right

    • Including water in the Ka expression.

    • Forgetting to take the square root of Ka × [HA] when finding [H⁺].

    • Giving pKa a unit, when taking a logarithm removes it.

    • Stating that a larger pKa means a stronger acid.

    • Using the acid concentration directly as [H⁺], as though the weak acid were strong.

    Exam tips

    • Set out the two approximations explicitly; some mark schemes award a mark for stating them.
    • Check the size of your answer: a weak acid at 0.1 mol dm⁻³ should give a pH around 3, not around 1.

    Try the lesson

    The start of the Weak acids and bases, Ka, for weak acids (A-level only): Ka is the dissociation constant for a weak acid. lesson, made from checked teaching notes for this specification point.

    Have a go first

    For the weak acid HA, which expression defines its acid dissociation constant Ka?

    Listen

    Ka is the equilibrium constant for the dissociation of a weak acid: for HA ⇌ H⁺ + A⁻, Ka = [H⁺][A⁻]/[HA], with units mol dm⁻³. Water does not appear, because it is the solvent. To find the pH of a weak acid on its own, two approximations are made: the H⁺ from water is negligible, so [H⁺] = [A⁻]; and so little acid dissociates that [HA] at equilibrium equals the concentration made up. Then Ka = [H⁺]²/[HA], so [H⁺] = √(Ka × [HA]). For butanoic acid, Ka = 1.51 × 10⁻⁵ mol dm⁻³, so a 0.100 mol dm⁻³ solution gives [H⁺] = 1.23 × 10⁻³ mol dm⁻³ and pH = 2.91. The logarithmic form pKa = −log₁₀Ka ranks acids compactly: a larger Ka, and so a smaller pKa, means a stronger acid. A strong answer earns marks for these points. the expression Ka = [H⁺][A⁻]/[HA]. the units mol dm⁻³. assuming [H⁺] = [A⁻] and that [HA] at equilibrium equals the initial concentration. pKa = −log₁₀Ka and the reverse conversion Ka = 10⁻ᵖᴷᵃ. Exam tip: Set out the two approximations explicitly; some mark schemes award a mark for stating them.

    See it

    A flowchart running from top to bottom. The top node reads Ka = [H⁺][A⁻]/[HA]. Three arrows lead from it. The first goes to a node reading Units mol dm⁻³. The second goes to a node reading Approximation: [H⁺] = [A⁻]. The third goes to a node reading Approximation: [HA] at equilibrium equals concentration made up. Both approximation nodes lead into a node reading Ka = [H⁺]²/[HA], which leads to a node reading [H⁺] = √(Ka × [HA]), which leads to a node reading pH. A separate arrow from the top node leads to a node reading pKa = −log₁₀Ka, which leads to a final node reading Larger Ka, smaller pKa means stronger acid.
    It shows the Ka expression and its units, the two approximations leading to [H⁺] = √(Ka × [HA]) and pH, and the pKa relationship used to rank acid strength.

    Key words

    Say what each word means before you turn the card.

    acid dissociation constant

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    approximations

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    pKa

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    ranking acid strength

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    See every definition
    acid dissociation constant
    The equilibrium constant for the dissociation of a weak acid, given by Ka = [H⁺][A⁻]/[HA], with units mol dm⁻³.
    approximations
    The assumptions that the H⁺ from water is negligible, so [H⁺] = [A⁻], and that so little acid dissociates that [HA] at equilibrium equals the concentration made up.
    pKa
    The logarithmic form pKa = −log₁₀Ka, where a smaller value means a stronger acid.
    ranking acid strength
    A larger Ka, and so a smaller pKa, means a stronger acid.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart running from top to bottom. The top node reads Ka = [H⁺][A⁻]/[HA]. Three arrows lead from it. The first goes to a node reading Units mol dm⁻³. The second goes to a node reading Approximation: [H⁺] = [A⁻]. The third goes to a node reading Approximation: [HA] at equilibrium equals concentration made up. Both approximation nodes lead into a node reading Ka = [H⁺]²/[HA], which leads to a node reading [H⁺] = √(Ka × [HA]), which leads to a node reading pH. A separate arrow from the top node leads to a node reading pKa = −log₁₀Ka, which leads to a final node reading Larger Ka, smaller pKa means stronger acid.

    • 1Box 1

    • 2Box 2

    • 3Box 3

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