Quick questions on Volumetric analysis and titration: VCE Chemistry Unit 2

A titration determines the concentration (or amount) of an analyte by reacting it with a known volume of a solution of known concentration (a standard solution) until the reaction is just complete (the equivalence point), signalled by a colour change at the endpoint.

A primary standard is a substance you can weigh directly and trust as the basis of an accurate solution. Criteria:

The shape of the titration curve depends on whether the acid and base are strong or weak.

Same workflow with a redox reaction instead of acid-base. The standard may be itself coloured (KMnO4 is intensely purple) and acts as its own indicator: the first persistent pink colour beyond the analyte signals the endpoint. Iodine titrations use starch (deep blue with I2). Acidified KMnO4 is used to titrate Fe^2+ and other reductants.

1. Calculate moles of titrant from c x V (use the mean titre and volumes in litres). 2. Convert to moles of analyte using the mole ratio from the balanced equation.

1. Add a known excess of a standard reagent that reacts completely with the analyte. 2. Titrate the unreacted excess against a second standard.

Always pull the ratio from a balanced equation. Sulfuric acid is diprotic: H2SO4 + 2NaOH -> Na2SO4 + 2H2O, so the ratio is 1:2.

c x V with V in mL gives moles 1000 times too large.

A weak acid titrated with a strong base has a basic equivalence point; phenolphthalein, not methyl orange.

NaOH absorbs water and CO2 from air. It must be standardised against a primary standard (oxalic acid or KHP).

Moles of analyte come from (initial standard) - (excess found by back-titration), then divided by the appropriate mole ratio.