Water boils when the pressure of its vapour above the liquid surface is equal to the atmospheric pressure. When substances are dissolved in water, the vapour pressure of the water is reduced and its boiling point is increased. The increase in boiling point is known as the boiling point elevation, ∆T, which is the difference between the boiling point of a solution and the boiling point of pure water. ∆T is usually small, often less than 1°C. When glucose is dissolved in 1 kg of water, the relationship between ∆T and the number of moles of glucose dissolved is as shown. ∆T = K_b × Z K_b is the boiling point constant of pure water Z = number of moles of glucose / mass of water, in kg in molkg⁻¹ A student carries out an experiment to determine the boiling point constant, K_b, for water. The student uses anhydrous glucose, C6H12O6, as the solute because it is non-volatile and very soluble in water. The experimental set-up the student uses is shown. [Figure showing digital meter, 99.48 °C, digital probe thermometer, reflux condenser, two-neck round-bottomed flask, 75.00 g distilled water, anti-bumping granules, heat] The student follows this procedure. 1 Transfer 75.00 g of distilled water to the round-bottomed flask. 2 Add anti-bumping granules to the distilled water to prevent violent, uneven boiling. 3 Heat the distilled water until it boils and record the highest stable temperature. 4 Stop heating and allow the distilled water to cool to room temperature. 5 Remove the reflux condenser and add about 1g of anhydrous glucose, measured accurately. 6 Replace the reflux condenser and heat the solution until it boils, noting the highest stable temperature. 7 Repeat steps 4 to 6, each time adding approximately 1g more of anhydrous glucose, accurately weighed, until sufficient readings are taken. The student constructed the table shown to record the results for this experiment. [Figure table with columns A (mass of glucose /g), B (boiling point /°C), C (amount of glucose in 75.00 g of water /mol), D (Z /molkg⁻¹), E (∆T /°C)] [Figure graph grid for plotting ∆T vs Z]

1. (a)Use the information above to explain why lowering the vapour pressure of a liquid increases the temperature at which it boils.[1]
2. (b)Show, using a labelled arrow, where the cooling water enters the reflux condenser.[1]
3. (c)A digital probe thermometer is used as shown in the diagram. Explain why a normal laboratory glass thermometer would not be suitable.[1]
4. (d)In step 4, the heating is stopped and the distilled water allowed to cool from its boiling point, before removing the reflux condenser. Apart from for safety reasons, explain why this is essential.[1]
5. (e)At 101 kPa (1 atm), distilled water is known to boil at 100.00 °C. Suggest why the boiling point of distilled water in this experiment was found to be 99.48 °C. Assume that the digital probe thermometer was reading correctly.[1]
6. (f)(i)Complete columns C and D to three significant figures and column E to two decimal places. [The M_r of glucose is 180.] Z = moles of glucose / mass of water, in kg[3]
7. (f)(ii)Plot a graph on the grid to show the relationship between ∆T and the amount of glucose in 1 kg of water, Z. Use a cross (x) to plot each data point. Draw a line of best fit.[2]
8. (f)(iii)Circle the most anomalous point on your graph.[1]
9. (g)Use the graph and the equation to determine the boiling point constant, K_b, for water. Give this value to three significant figures and state the units. ∆T = K_b × Z State the co-ordinates of both points you used in your calculation. co-ordinates 1 co-ordinates 2 K_b = units =[3]
10. (h)(i)The experiment was repeated using solid from a bottle that was labelled glucose, C6H12O6, but actually contained sucrose, C12H22O11. Sucrose is non-volatile and very soluble in water. What would be the effect of this on the value the student obtained for K_b? Explain your answer.[1]
11. (h)(ii)The student used distilled water to dissolve the glucose. Suggest why the student did not use tap water.[1]
12. (h)(iii)The student repeated this experiment using sodium chloride as the solute. The student found their calculated value of K_b was twice the calculated value of that obtained with glucose. Suggest a reason for this.[1]

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