1: To use a standard solution of anhydrous sodium carbonate to standardise a solution hydrochloric acid

Theory

By titrating a provided hydrochloric acid (HCl) solution of unknown concentration against a standard solution of sodium carbonate (Na2CO3), the concentration of the hydrochloric acid can be determined.

Equation

Na2CO3 + 2HCl → 2NaCl + H2O + CO2

Procedure

1) Filling the burette with the hydrochloric acid solution to be standardised

2) Making the anhydrous sodium carbonate up into a standard solution

3) Transferring an exact volume of sodium carbonate solution into a conical flask

4) Carrying out the titration

Suitable indictor for this titration

Methyl orange

Justification for indicator

• It is a strong acid-weak base titration, the end point occurs below a pH of 7

• Methyl orange changes colour between pH 3 and 5 coinciding with this end point

Colour change

Yellow → pink

(Base) → (Acid)

Explain why a standard solution of hydrochloric acid CANNOT be directly made up i.e. why must

hydrochloric acid be standardised by titration?

Hydrochloric acid is not a primary standard

Explain why a standard solution can be directly made up from anhydrous sodium carbonate

Anhydrous sodium carbonate is a primary standard - it is pure, is stable in air, has a high molar mass and can dissolve easily in water to directly make up a solution of accurately known concentration (a standard solution)