Question

The 1.2 liter coffee pot is filled with 15 ° C water. What is the amount of heat used to heat and boil water when 50 cm3 of water is known to have evaporated? (Do not ignore the changed water density as the temperature changes. Water density 1000 kg / m3).

Answer 1

When it is noted that we cannot ignore the variation of density with temperature it is meant to say that while calculating heat used in vaporisation at 100^oC we cannot take density of water to be 1000kg/m³ We have to use the real value of density at 100^oC which is 958kg/m³.

Now all through the process we can see the heat being spent in two ways.One is spent to rise the temperature of water from 15 to 100. Then other is vaporising the 50cm³ of water.

First let us calculate first one:

As mentioned already for this calculation we can use density of water to be 1000kg/m³. Also specific heat capacity of water,C =4.2 KJ/Kg°C. Now,We use

Now let us calculate heat of vaporisation. Here we will have to use exact density of water for calculation of mass.

We know latent heat of vaporisation,L_v = 2260 kJ/kg. Now:

So total heat used E_t is given by:

SO heat used = 537.654 KJ