(a) In an experiment to measure the specific latent heat of vapourisation of water, a...
(a) In an experiment to measure the specific latent heat of vapourisation 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 were taken every 100s when the water starts boiling.
The table below shows the readings.
| Time/s | 0 | 100 | 200 | 300 | 400 |
| Reading on balance/g | 203.22 | 201.62 | 199.79 | 198.26 | 196.50 |
| Mass of water evaporated/g | 0 | ||||
| Energy supplied by heated/J | 0 |
(I) Fill in the mass of water evaporated.
(ii) Given that the heater supplies energy at the rate of 38J/s, fill in the values of the energy supplied by the heater in 100s, 200s, 300s, and 400s.
(ii) 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).
(iv) Determine the slope of the graph.
(v) what does the value of the slope mean?
Explanation
(a)(i)
| Time/s | 0 | 100 | 200 | 300 | 400 |
| Reading on balance/g | 203.22 | 201.62 | 199.79 | 198.26 | 196.50 |
| Mass of water evaporated/g | 0 | 1.60 (\(\frac{1}{2}\)) | 3.43 (\(\frac{1}{2}\)) | 4.96 (\(\frac{1}{2}\)) | 6.72 (\(\frac{1}{2}\)) |
| Energy supplied by heated/J | 0 | 3800 (\(\frac{1}{2}\)) | 7600 (\(\frac{1}{2}\)) | 11400 (\(\frac{1}{2}\)) | 15200 (\(\frac{1}{2}\)) |
(iv)
Determination of slope of graph
Slope = \(\frac{\Delta Q}{\Delta M}\)
= \(\frac{14.0 - 2.0) \times 10^3}{6.1 - 0.8}\)
= 2.26 x 10\(^3\)J/g
(v) Meaning of the value of the slope of graph
2.26 x 10\(^2\)J of heat energy is needed to evaporate 1g of water at constant temperature.
(b)(i) Explanation of saturated vapour pressure;
At any given temperature, some energetic molecules of a liquid in a given system escape from the liquid surface as vapour and at the same time some return to the liquid. Vapour pressure is built up above the liquid in the process. When the rate of escape equals the rate of return, the vapour is said to be saturated and the pressure it exerts is called saturated vapour pressure at that temperature.
(ii) Factor that affects saturated vapour pressure;
- Temperature