paper 3 - practical skills
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A student has an unknown weak acid of unknown concentration and a bottle of 0.5 mol dm-3 NaOH. Explain, using brief practical details, how you would determine the Ka of the weak acid. [6]
• Measure 25 cm3 of acid into a beaker/conical flask using volumetric pipette. • Use a pH probe to measure the initial pH • Add 1cm3 NaOH from a burette swirl and record the pH • Repeat until the NaOH is in excess • Plot a graph of pH against vol NaOH added in cm3 • Use the vertical section to find the volume of NaOH needed for neutralisation • The half neutralisation point is half the volume of neutralisation. • Read off the pH at the half neutralisation point. • At half neutralisation ka = [H+]/pKa=pH • Ka = 10-pH