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Module 6: Acid/Base Reactions
Quick questions on Conjugate acid-base pairs (Ka, Kb, Kw relationship) explained: HSC Chemistry Module 6
10short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is the inverse relationship?Show answer
When an acid ionises, its conjugate base forms:
What is salt hydrolysis?Show answer
A salt is named by its cation and anion. Each ion comes from an acid or a base.
What is calculating the pH of a salt solution?Show answer
For a salt of a strong base and a weak acid (say, sodium ethanoate at concentration $c$):
What is step 1: Identify the hydrolysing ion?Show answer
$NH_4^+$ is the conjugate acid of $NH_3$ (weak base). $Cl^-$ is a spectator (conjugate base of strong acid).
What is treating $Cl^-$ as a weak base?Show answer
$Cl^-$ is the conjugate of a strong acid. Its $K_b$ is too small to influence pH. Do not write a hydrolysis equation for it.
What is forgetting that $Na^+$ and $K^+$ are spectators?Show answer
They never affect pH at HSC.
What is mixing up $K_a$ and $K_b$ in the identity?Show answer
$K_a$ refers to the acid; $K_b$ refers to its conjugate base. Use $K_a \cdot K_b = K_w$ on the same pair only.
What is using $K_b$ of $NH_3$ when you mean $K_a$ of $NH_4^+$?Show answer
They are different sides of the same conjugate pair. When you have a salt like $NH_4Cl$, the ion in solution is $NH_4^+$, so you need its $K_a$. Convert with $K_w / K_b$.
What is ignoring the second hydrolysis of $CO_3^{2-}$?Show answer
Carbonate is the conjugate base of $HCO_3^-$ (not $H_2CO_3$ directly). Use $K_a$ of $HCO_3^-$ ($K_{a2}$ of carbonic acid, about $4.7 \times 10^{-11}$). A common slip is to use $K_{a1}$ instead.
What is forgetting to take square root?Show answer
The weak-acid or weak-base ICE shortcut gives $x = \sqrt{K \cdot c}$, not $K \cdot c$.