Back Titration Concept and Method

Overview

This lecture explains the concept of back titration and demonstrates how to solve calculation questions using the model method with an example.

Back titration is used when the analyte cannot be titrated directly or reacts slowly with the titrant.
An excess known amount of reagent B is added to react completely with analyte A.
The remaining unreacted B is then titrated with a second reagent, the titrant T.
Calculations involve working backwards to determine the original amount of analyte A.

There are two key reactions: B reacts with A, then unreacted B is titrated with T.
First, calculate the amount of titrant T used to react with the excess B.
Next, find the amount of excess B left by using titration data.
Subtract excess unreacted B from the total added B to find B that reacted with A.
Use stoichiometry from the balanced equation to relate amounts and find the amount or mass of A.

Identify reagents: B is the excess (hydrochloric acid), T is the titrant (sodium hydroxide), and A is the analyte (magnesium carbonate).
Write out balanced equations for both reactions.
Convert titrant volume to moles (e.g., 36.8 mL NaOH × 0.1 mol/dm³).
The moles of titrant used equal the moles of excess B (HCl) reacted.
Total initial moles of B are calculated from the given volume and concentration.
Subtract moles of unreacted B (from titration) from total B to get moles that reacted with A.
Use the stoichiometric ratio (from the balanced equation) to calculate moles of A.
Multiply moles of A by its molar mass to find its mass.

Back Titration — Analytical method where excess reagent is reacted with the analyte and residual excess is titrated.
Analyte (A) — The substance in unknown amount to be determined.
Excess Reagent (B) — The known chemical added in excess to react with analyte.
Titrant (T) — The standard solution used to titrate the excess reagent.