AQA GCSE Chemistry Higher Tier Paper 2

A student investigated an aqueous solution of a salt. The student identified that the salt solution contained only sodium ions and chloride ions.

Describe a test to identify sodium ions. Give the result of the test.

Describe a test to identify chloride ions. Give the result of the test.

The student determined the concentration of sodium chloride in the salt solution using this method:

1. Weigh an empty evaporating dish.
2. Add 25.0 cm³ of the salt solution into the evaporating dish.
3. Heat the evaporating dish and contents.
4. Weigh the evaporating dish and contents.
5. Repeat steps 3 to 4 until there is no further change in mass.
6. Repeat steps 1 to 5 three more times.

Why did the student heat the evaporating dish and contents until the mass did not change?

How did the student calculate the mass of solid sodium chloride remaining after steps 1 to 5?

The student calculated the concentration of sodium chloride in the salt solution. Table 1 shows the results.
The percentage by mass of sodium ions in sodium chloride is 39.3%.
Calculate the mean concentration of sodium ions in the salt solution.