First slide

First law of thermodynamics

Question

1 g of water, of volume $1 {cm}^{3}$ at 100°C, is converted into steam at same temperature under normal atmospheric pressure $(1 \times 10^{5} Pa) .$ The volume of steam formed equals $1671 {cm}^{3}$ . If the specific latent heat of vaporisation of water is 2256 J/g, the change in internal energy is,

Difficult

A

2256 J
B

2423 J
C

2089 J
D

167 J

Solution

$ΔQ = 2256 \times 1 = 2256 J ΔW = P [V_{steam} - V_{water}] \begin{array}{l} = 1 \times 10^{5} [1671 - 1] \times 10^{- 6} \\ = 1670 \times 10^{5} \times 10^{- 6} \\ = 167 J \end{array}$
By first law of thermodynamics:
$As ΔQ = ΔU + ΔW$
$2256 = ΔU + 167$
$Δ U = 2089 J$

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