Saturday, 6 September 2014

Latent heat problems


                           Latent heat problems

1. A shiny electric kettle with a 2.0 kW heating element has a thermal capacity of 400 JK-1. 1.00 kg of water at 20oC is placed in the kettle. The kettle is then switched on and 0.5 kg of water remain after 13 minutes. Ignoring heat losses, calculate the value for the specific latent heat of vaporisation of water.
(Specific heat capacity of water = 4200 J kg-1K-1)

If heat losses had been taken into account, how would your answer be modified?

2. An ice cube whose mass is 50 g is taken from a fridge at -10oC and dropped into a glass of water at 0oC. If no heat is gained or lost from the outside, how much ice will freeze on to the cube? 
(Specific latent heat of fusion of water = 330 000 Jkg-1

3. 2 g of iron wire at 15 °C are dropped into liquid oxygen maintained at its boiling point in a vacuum flask. The volume of oxygen driven off, measured at 20oC and 1.2 x 105 Pa, is 
4.32 x 10-4 m3.
Find the specific latent heat of vaporisation of oxygen.
(Specific heat capacity of iron = 106 J kg-1K-1; boiling point of oxygen = -184o-3)

4. A student measures the specific latent heat of ethanol using an electrical method as described in the experiments section. He makes two measurements, the results of which are given below.
Experiment 1: heater voltage 8.90 V, heater current 2.10 A, mass of ethanol evaporated 174 g in 15 minutes.
Experiment 2. heater voltage 7.30 V, heater current 1.74 A, mass of ethanol evaporated 111 g in 15 minutes.

Calculate the specific latent heat of ethanol from these results.