An electric heater with constant heat supply rate is used to convert a certain amount of liquid ammonia to saturated vapour at high pressure. The heater takes 14 minutes to bring the liquid at `15^@C` to the boiling point of `50^@C` and 92 minutes to convert the liquid at the boiling point wholly to vapour. If the specific heat capacity of liquid ammonia is 4.9 kJ/kg K, the latent heat of vaporisation of ammonia in kJ/kg is

Options

A. 557
B. 981
C. 1127
D. 2250

Answer

C

Explanation

Let mass of liquid ammonia taken is m and rate of heat supplied is r .
Then, as per given condition
`r xx14 = m xx 4.9 x (50 - 15) " and " r xx 92 = mL`
Dividing these, we get
` 92/14 = (L)/(4.9 xx (50 - 15) ) " or " L = (92 xx 4.9 xx 35)/(14)`
L=1127 kJ/kg