Edexcel IAL Chemistry 14.8 Acid dissociation constant expressions

Identify the acid and conjugate base from the formula or structure, then place aqueous concentrations in square brackets without losing ionic charges.

Exam points

Write Ka as [H+][A-]/[HA] or the equivalent expression for the named weak acid.
Preserve charges and formulas for conjugate bases such as carboxylates or protonated amines.
Complete the dissociation equation before translating the species into the Ka expression.

[Maximum number: 1]

The alkaline compound tris(hydroxymethyl)aminomethane, known as Tris, is used to make a buffer for biological research.

Tris

Tris is a Brønsted-Lowry base and its conjugate acid is formed as shown.

Figure for Question 14.8—Deduce the expression for the acid dissociation constant, Ka, for a weak acid question 1 — Edexcel A-Level Chemistry A2

Write the expression for the $K_{a}$ of the conjugate acid of Tris $\left(\mathrm{C}_{4} \mathrm{H}_{12} \mathrm{NO}_{3}^{+}\right)$ .

correct expression

Allow use of skeletal structure or molecular formula
$K_{\mathrm{a}}=\frac{\left[\mathrm{C}_{4} \mathrm{H}_{11} \mathrm{NO}_{3}\right]\left[\mathrm{H}^{+}\right]}{\left[\mathrm{C}_{4} \mathrm{H}_{12} \mathrm{NO}_{3}^{+}\right]}$

Do not accept round brackets

NB If no answer is given here, check in the answer to 22(b)(iii)

(1)

Figure for Answer to Question 14.8—Deduce the expression for the acid dissociation constant, Ka, for a weak acid question 1 — Edexcel A-Level Chemistry A2

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14.8—Deduce the expression for the acid dissociation constant, Ka, for a weak acid question 1