Question

11(a) In an experiment to measure the specific latent heat of vapourization of water, a student places a heater in a beaker containing water. The beaker stands on an electronic balance so that the mass of the beaker and water could be measured. The heater is switched on and readings taken every 100 s when the water starts boiling. The table below shows the reading (Time(s): 0,100,200,300,400 | Reading on balance(g): 203.22, 201.62, 199.79, 198.25, 196.50). (i) Fill in the mass of water evaporated. (ii) Given that the heater supplies energy at the rate of 38 J/s, fill in the values of the energy supplied by the heater in 100 s, 200 s, 300 s and 400 s. (iii) Plot a graph of energy supplied on the vertical axis and mass of water evaporated on the horizontal axis, starting both axes from the origin (0,0). I. Determine the slope of the graph. II. What does the value of the slope mean? (b)(i) Explain what is meant by saturated vapour pressure. (ii) State the factor that affects saturated vapour pressure.

Accepted Answer

(a)(i) Mass of water evaporated = (New mass) - (Initial mass), calculated at each time interval from the balance readings: at 100s, mass evaporated=203.22-201.62=1.60g; at 200s, =203.22-199.79=3.43g; at 300s,=203.22-198.25=4.96g; at 400s,=203.22-196.50=6.72g.
(ii) Energy supplied by heater = rate of energy supply x time = (38 J/s) x time. At 100s: 3800J; at 200s: 7600J; at 300s: 11,400J; at 400s: 15,200J (values filled based on the given rate of 38 J/s, consistent with the table's progression pattern of 0, 38.00, 76.00, 114.00, 152.00 in the units indicated in the table, i.e. in units of 10^2 J).
(iii) I. Slope of the graph = (Delta Q)/(Delta m) = (Q2-Q1)/(m2-m1) = (11.2-2.0)x10^3 / (5.0-0.9) = 9.2x10^3/4.1 = 2.24x10^3 J/g.
II. Meaning of the value of the slope of a graph: The slope = 2.24x10^3 J of heat is needed to evaporate 1 g of water at constant temperature; this represents the specific latent heat of vaporization of water.
(b)(i) Saturated vapour pressure: The process of evaporation in a closed container will proceed until there are as many molecules returning to liquid as there are escaping. At this point, the vapour is said to be saturated. This is called the saturated vapour pressure of the liquid at that temperature.
(ii) Factor affecting saturated vapour pressure: Temperature. Vapour pressure increases as the temperature increases and vice-versa.

WAEC Physics 2020 Theory — Question 11

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