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NSWChemistryQuick questions
Module 5: Equilibrium and Acid Reactions
Quick questions on pH and pOH calculations explained: HSC Chemistry Module 5
11short 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 pH scale?Show answer
A lower pH means a higher $[H^+]$ and a more acidic solution. Each unit of pH corresponds to a tenfold change in $[H^+]$.
What is the auto-ionisation of water?Show answer
$$2H_2O_{(l)} \rightleftharpoons H_3O^+_{(aq)} + OH^-_{(aq)}$$
What is strong acids and bases?Show answer
Strong acids (HCl, $HNO_3$, $H_2SO_4$, $HClO_4$) dissociate completely. $[H^+]$ equals the acid concentration (for monoprotic acids).
What is weak acids and bases?Show answer
Weak acids dissociate only partially. Use the dissociation constant Ka and an ICE table.
What is significant figures for logs?Show answer
Only the digits after the decimal point in a log are significant. A $[H^+]$ of $1.64 \times 10^{-3}$ (3 sig fig) gives pH = 2.79 (2 decimal places).
What is step 2: Check the 5% rule?Show answer
$4.24 \times 10^{-4} / 0.0010 = 42\%$, well above 5%. The approximation fails. Solve the quadratic.
What is forgetting to double for $Ca _2$ or $Ba _2$?Show answer
Each formula unit produces two $OH^-$.
What is mixing $H_2SO_4$ assumptions?Show answer
The first dissociation is complete; the second has Ka ≈ $1.2 \times 10^{-2}$. HSC usually treats $H_2SO_4$ as fully dissociating both protons for dilute solutions, but a careful answer states the assumption.
What is skipping the 5% check for weak acids?Show answer
If $x$ exceeds 5% of $C_0$, you must solve the quadratic. The approximation fails for concentrations below ~$10^{-3}$ mol/L of typical weak acids.
What is wrong sig figs?Show answer
For pH, the number of decimal places equals the sig figs of the concentration. pH = 2.79 (3 sig fig), not 2.794 or 2.8.
What is dilution errors?Show answer
When diluting a strong acid 10-fold, pH increases by 1 (more dilute, less acidic). When diluting a weak acid 10-fold, pH increases by about 0.5 because dissociation increases as concentration falls.