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QLDChemistryQuick questions
Unit 3: Equilibrium, acids and redox reactions
Quick questions on pH, Kw and the self-ionisation of water (QCE Chemistry Unit 3)
12short 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 self-ionisation of water?Show answer
Two water molecules can transfer a proton:
What is the pH definition?Show answer
Taking the log of Kw at 25 degrees C:
What is strong acid pH calculations?Show answer
For monoprotic strong acids (HCl, HBr, HI, HNO3, HClO4), [H3O+] = [acid] for any concentration above about 10^-6 mol/L.
What is strong base pH calculations?Show answer
For sodium hydroxide and other group 1 hydroxides, [OH-] = [base]. Calculate pOH then convert to pH.
What is mixed strong acid + strong base solutions?Show answer
Three steps: 1. Calculate moles of H3O+ and OH- from the original solutions. 2. Identify which is in excess; the smaller amount is consumed completely.
What is dilution calculations?Show answer
Diluting a strong acid by a factor of f decreases [H3O+] by f, so pH rises by log10(f).
What is temperature dependence of Kw?Show answer
Self-ionisation is endothermic. As T rises: - Kw increases. - Both [H3O+] and [OH-] rise in pure water. - Pure water's pH falls below 7 (5.86 at body temperature, around 6.5 at 60 degrees C).
What is significant figures and pH?Show answer
A pH value has digits after the decimal that correspond to the significant figures of the [H3O+] value. pH 1.6 (one digit after decimal) corresponds to [H3O+] known to one significant figure. pH 1.60 corresponds to [H3O+] known to two significant figures.
What is using initial volumes when calculating concentration after mixing?Show answer
Once mixed, the new volume is the sum (assume volumes add).
What is treating weak acid solutions as if [H3O+] = [acid]?Show answer
This dot point covers strong acids and bases. For weak acids you need Ka and an ICE setup (covered as part of the broader Topic 1 in some schools and in the buffer dot point here).
What is reporting pH with too many significant figures?Show answer
Match decimal places to the significant figures of the concentration.
What is assuming pH 7 is always neutral?Show answer
Neutrality requires [H3O+] = [OH-], which gives pH 7 only at 25 degrees C.