Question

It takes 15 minutes for an electrical kettle to heat a certain quantity of water in grams from $0^{o}C$ to boiling point ${100}^{o}C$. If it takes 81 minutes to boil water into steam, the latent heat of steam is :

• A. $80cal/g$
• B. $540cal/g$
• C. $100cal/g$
• D. $336cal/g$

Answer 1

Answer: (B)

$540cal/g$

Suppose mass of water = m
Heat required to raise its temperature from 0${}^o$ C to 100${}^o$ C is 100m
$\therefore$ Heat produced by the kettle in 15 minutes $=$ 100m
So, feat produced by the electric kettle in one minute $=\frac{100m}{15}$

And, heat produced by the electric Kettle in 81 minutes $=\frac{100m\times 81}{15}cal$
Suppose Latent heat of steam $=$ L cal/gm
Heat required by m grams of water to boil off into steam $=mLcal$
$\therefore mL=\frac{100m\times 81}{15}$
$L=540cal/gm$