Question

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 $15^{\circ} C$ to the boiling point of $50{ }^{\circ} C$ 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

• A. 557
• B. 981
• C. 1127
• D. 2250

Answer 1

Answer: (C)

Let mass of liquid ammonia taken is $m$ and rate of heat supplied is $r$.
Then, as per given condition,
$r \times 14=m \times 4.9 \times(50-15)$ and $r \times 92=m L$
Dividing these, we get
$\frac{92}{14} =\frac{L}{4.9 \times(50-15)}$ or $L=\frac{92 \times 4.9 \times 35}{14}$
$=1127\, kJ / kg$