Titration question.

Question

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Samples of hydrochloric acid, of unknown concentration, were titrated with a solution of sodium carbonate. The flasks were rinsed with sodium carbonated stock solution before the samples of hydrochloric acid were added. Explain what effect this would have on the calculated concentration of acid.

Answer 1

Hydrochloric acid is HCl

Sodium carbonate is Na₂CO₃

The reaction that takes place between the two is 2HCl + Na₂CO₃ ==> 2NaCl + H₂O + CO₂

If the flasks were rinsed with Na₂CO₃ prior to the addition of HCl, the residual Na₂CO₃ left in the flask would react with some of the HCl, making the effective moles of HCl remaining LESS than the investigator would think.  This in turn would result in using LESS Na₂CO₃ for the titration, and this would thus REDUCE the calculated concentration of the acid.