Question

5. In this experiment, you made calculations beforehand to determine amounts needed in the procedure. Some information was provided to help you make these calculations. Imagine that you are working in a laboratory and you need to find the concentration of an unknown sample of acetic acid, and you have no idea whatsoever what its concentration might be. What preliminary steps might you take to plan a titration that would accurately determine the concentration? (Consider: how might you decide what concentration of NaOH to use? How would you decide if you should dilute the sample or not?)

Answer 1

We know that acteic acid is a weak acid. It will react with strong base such as NaOH in a neutralization reaction to give salt and water.

Acid + Base ------> Salt + Water

CH₃COOH + NaOH ----------> CH₃COONa + H₂O

The preliminary steps to be taken for the titration are as follows:

1. Take a known amount of acetic acid sample (10ml) and weigh it accurately on a weighing balance.

2. Calculate the density of acetic acid by the formula ===> Density = Mass/ volume.

3. Prepare a 1 Molar solution of NaOH by dissolving  40 gm of NaOH in 1000 mL of water.

4. If the density of acetic acid is more than 1 then dilute 1 mL of acetic acid to 100mL in a 100 mL dilution flask.

5. Take 25mL of this acetic acid solution in a conical flask and add phenolphthalein indicator to it.

6. Titrate with 1M NaOH solution from the burette till the colourless solution in the flask turns into dark pink coloured solution which indicates the end point of the titration.

7. Calculate the number of moles of NaOH used in the titration by using the formula:

Moles of NaOH = x L of NaOH added x 1 mole of NaOH/ 1000L of NaOH solution

The acteic acid combines with NaOH in a 1:1 ration

Hence Moles of acetic acid in the solution = moles of NaOH used in the titration

no of gms of acetic acid = no. of moles of acetic acid x molecular weight of acetic acid (60)

To calculate the %weight of acetic acid = weight in gms of acetic acid / 25 gm