When anhydrous sodium carbonate dissolves in water the following reaction occurs. Na2CO3(s) + aq → 2Na+(aq) + CO32-(aq) The enthalpy change for this reaction is the enthalpy change of solution of anhydrous sodium carbonate. Its value is -28.1 kJ mol-1. You will use this information to find the percentage purity of an impure sample of anhydrous sodium carbonate. FB 3 is a sample of impure anhydrous sodium carbonate.

1. (a)Method Experiment 1 • Weigh the cup and record the mass. • Place between 1.9g and 2.1 g of FB 3 in the cup. Record the mass of the cup + FB 3. • Calculate and record the mass of FB 3 used. • Support the cup in the beaker. • Place 25.0 cm³ of distilled water into the measuring cylinder. Measure and record the temperature of the water. • Place the water in the cup and stir until all the solid dissolves. • Place the thermometer in the solution and record the highest temperature reached. Tilt the cup if necessary so that the bulb of the thermometer is fully covered. • Rinse the cup and shake dry ready to carry out Experiment 2. Experiment 2 • Repeat the experiment using between 3.9g and 4.1 g of FB 3 and 25.0 cm³ of water. • Record your results in the same way as in Experiment 1. Results I II III IV V[5]
2. (b)(i)Use your results from Experiment 1 to calculate the heat energy, in J, released when the FB 3 dissolves. (Assume 4.2 J of heat energy changes the temperature of 1.0 cm³ of solution by 1.0°C.) heat energy = ...................................... J[1]
3. (b)(ii)Use the information on page 4 to calculate the number of moles of anhydrous sodium carbonate needed to release the amount of energy in your answer to (b)(i). moles of Na2CO3 = ...................................... mol[1]
4. (b)(iii)Calculate the mass of anhydrous sodium carbonate present in the number of moles in your answer to (b)(ii). mass of Na2CO3 = ...................................... g[1]
5. (b)(iv)Use your answer to (b)(iii) to calculate the percentage purity of anhydrous sodium carbonate in FB 3. percentage purity = ......................................[1]
6. (c)In (b)(iv) you calculated the percentage purity of anhydrous sodium carbonate in FB 3. What did you assume about the impurity present?[1]
7. (d)You carried out two experiments to measure the temperature change when anhydrous sodium carbonate was dissolved in water. Complete the sentence below to explain which of these experiments was more accurate. Your answer should not make reference to the fact that different masses of sodium carbonate were used. Experiment .............................. was more accurate because ..............................[1]