Question

If $2$kg of steam at ${100}^o$C condenses into water at ${40}^o$C the energy evolved in calories is $(L=540cal{g}^{-1},s=1cal{g}^{{-1}^o}{C}^{-1})$.

Answer 1

Mass of the steam = 2kg

At 100℃

L = 540cal/g.

$\Rightarrow$ = 54000cal/kg.

steam water

T = 100℃ $\to$ T = 100℃

$Q_1$ $\downarrow$$Q_2$

Water

T = 40℃

$Q_1$ = mL

$\Rightarrow$ = 2 $\times$54000

$\Rightarrow$ = 108000cal

$△$T = 100- 40

$\Rightarrow$ =${60}^0$

$\therefore$ $Q_{12}$ = ms$△$T

$\Rightarrow$ = $2\times 1\times 100\times 60$

$\Rightarrow$ = 12000cal.

Energy evolved :

$\Rightarrow$ = $Q_1$+$Q_2$

$\Rightarrow$ = 12000+108000

$\Rightarrow$ = 120000cal

$\Rightarrow$ = 1200kcal.