Edexcel IAL Chemistry Arithmetic and Weighted Means

Washing soda is hydrated sodium carbonate ( $\mathrm{Na}_{2} \mathrm{CO}_{3} \cdot \mathrm{nH}_{2} \mathrm{O}$ ). The value of n can be determined by titration.

A titration method was carried out using the following procedure.
Step 14.29 g of $\mathrm{Na}_{2} \mathrm{CO}_{3} \cdot \mathrm{nH}_{2} \mathrm{O}$ was weighed into a beaker.
Step 2 The $\mathrm{Na}_{2} \mathrm{CO}_{3} \cdot \mathrm{nH}_{2} \mathrm{O}$ was dissolved in $50 \mathrm{~cm}^{3}$ of deionised water.
Step 3 All of the solution was transferred to a $250 \mathrm{~cm}^{3}$ volumetric flask. The beaker was rinsed with deionised water and these washings were added to the volumetric flask.

Step 4 The volumetric flask was made up to the mark with deionised water. The flask was stoppered and inverted several times.

Step 5 A burette, pipette and conical flask were all rinsed with deionised water.
Step 6 The burette was rinsed and then filled with $0.125 \mathrm{~mol} \mathrm{dm}^{-3}$ hydrochloric acid solution. The pipette was rinsed with the sodium carbonate solution and then used to transfer $25.0 \mathrm{~cm}^{3}$ of the solution to the conical flask.

Step 7 A few drops of methyl orange were added to the conical flask. The sodium carbonate solution was titrated with the hydrochloric acid. This step was repeated until concordant titres were obtained.

The following results were obtained in the titration.

Complete the table and calculate the mean titre, using all concordant values.