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VICChemistryQuick questions
Unit 2: How do chemical reactions shape the natural world?
Quick questions on Strong vs weak acids and bases, Ka and Kb: VCE Chemistry Unit 2
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 what "strong" and "weak" mean?Show answer
A strong acid ionises essentially completely in water. Almost every $HA$ molecule donates its proton.
What is strong is not the same as concentrated?Show answer
Strong vs weak describes the extent of ionisation. Concentrated vs dilute describes the amount per litre. These are independent. A weak acid can be concentrated (glacial ethanoic acid is $17\ mol\ L^{-1}$ and still weak).
What is the acid ionisation constant Ka?Show answer
For the general weak-acid ionisation $HA + H_2O \rightleftharpoons H_3O^+ + A^-$, the equilibrium constant (with water activity absorbed into $K_a$) is:
What is the base ionisation constant Kb?Show answer
For a weak base $B + H_2O \rightleftharpoons BH^+ + OH^-$:
What is the Ka, Kb and Kw relationship?Show answer
For any conjugate acid-base pair in water at 25 deg C:
What is why the weak acid pH is higher than the strong acid pH at the same concentration?Show answer
Two solutions of $0.10\ mol\ L^{-1}$ acid:
What is confusing $K_a$ with $[H^+]$?Show answer
$K_a$ is a constant for a given acid at a given temperature. $[H^+]$ depends on both $K_a$ and the concentration of the acid.
What is using a one-way arrow for a weak acid?Show answer
Weak acids must be drawn with equilibrium arrows ($\rightleftharpoons$).
What is forgetting that $K_w = K_a \times K_b$?Show answer
A common SAC trap: given $K_a$, find $K_b$ of the conjugate. Divide $K_w$ by $K_a$.
What is listing $HF$ as a strong acid?Show answer
All other hydrogen halides ($HCl$, $HBr$, $HI$) are strong. $HF$ is weak (the $H-F$ bond is unusually strong).