3: To determine the concentration of ethanoic acid in vinegar

Theory

• Vinegar is a solution of ethanoic acid (CH3COOH)

• The concentration of the ethanoic acid in vinegar can be determined by titrating a diluted version of the vinegar against a previously standardised solution of sodium hydroxide (NaOH)

Equation

CH3COOH + NaOH → CH3CCONa + H2O

Procedure

1) Diluting the vinegar

2) Filling the burette with the diluted vinegar

3) Making the standard solution of sodium hydroxide up into a solution

4) Transferring an exact volume of sodium hydroxide solution into a conical flask

5) Carrying out the titration

Suitable indictor for this titration

Phenolphthalein

Justification for this indicator

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

• Phenolphthalein changes colour between pH 8-10 coinciding with this end point

Colour change observed at end point

Pink → Colourless

(Base) → (Acid)

What acid is present in vinegar and what is its chemical formula?

Ethanoic acid - CH3COOH

Identify the acid that occurs in nettles and stinging ants and what is its chemical formula

Methanoic acid - HCOOH

To what family of organic compounds do methanoic acid and ethanoic acid belong?

Carboxylic acids

A type of vinegar is white wine vinegar. What compound in white wine is converted to ethanoic acid in vinegar? What type of chemical process is responsible for this conversion

• Ethanol

• Oxidation (converts primary alcohols to aldehydes and then to carboxylic acids)

Why was the vinegar diluted before being used in a titration?

• The vinegar was diluted as the original vinegar is too concentrated meaning the end point of the titration would occur very quickly

• The titre value would be very small – smaller values have higher % error and are less accurate

Starting with a primary standard solution made from anhydrous sodium carbonate, what two titrations are required to standardise a sodium hydroxide solution

1) A titration of the standard solution of anhydrous sodium carbonate against a hydrochloric acid solution (titration 1)

2) A titration of the now standardised hydrochloric acid solution against the sodium hydroxide solution (titration 2)