Acids and Bases qp
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Propanoic acid is a weak acid. State the meaning of weak in this context.
partially dissociates (into ions) when in an aqueous solution)
Suggest why a student doing an experiment to produce the curve in the figure above would add the sodium hydroxide solution dropwise around the equivalence point.
As there is a large pH change (for a small addition of alkali)
Give an expression for Ka for propanoic acid (CH3CH2COOH). Use this expression to show that pH = pKa when half of the propanoic acid has reacted with sodium hydroxide.
Ka= [H+][CH3CH2COO-]/[CH3CH2COOH] [CH3CH2COOH] = [CH3CH2COO-] [H+]=Ka
When sodium hydroxide solution is added to aqueous propanoic acid, the solution formed acts as a buffer when between 5 cm3 and 15 cm3 have been added. Explain why the pH stays approximately constant during this part of the experiment.
M1 OH- reacts with propanoic acid OR reacts with H+ M2 EITHER ratio of [CH3CH2COOH] to [CH3CH2COO- ] remains almost constant OR Equilibrium for dissociation of CH3CH2COOH moves right to maintain [H+] /replace H+
Methyl orange and universal indicator are not suitable indicators for the titration of solutions of propanoic acid with sodium hydroxide. State the reason why each indicator is not suitable.
M1 Methyl orange - would not change colour at the equivalence point (allow end-point)/pH range does not match rapid pH change/pH range does not include/match equivalence point Universal indicator - idea of range of colours during titration/no distinct colour change (at equivalence/end-point)
A student plans to titrate butanoic acid solution with a solution of ethylamine. Explain why this titration could not be done using an indicator.
M1 This is a weak acid and weak base/alkali titration M2 pH change is too gradual/not sharp (at the equivalence point so colour change of indicator is difficult to judge)