25 The vapour pressure of water is 17.54 mm Hg at 293K. Calculate vapour pressure of 0.5 molal solution of a solute in it.

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0.5 molal solution contains 0.5 mol of solute in ( 1000 mathrm{ml} ) of water vapour pressure of pure water ( , mathrm{P}^{circ}= ) ( 17.54 mathrm{mm} mathrm{hg} ) no. of moles of water ( =1000 mathrm{g} / 18= ) 55.55 mol no. of moles of solute ( =0.5 mathrm{mol} ) hence mole fraction of water ( =55.55 / ) ( (55.55+0.5)=55.55 / 56.05=0.99 ) mole fraction of solute ( =1-0.99=0.01 ) vapour pressure of water ( , mathrm{P}=mathrm{P}^{circ} mathrm{Xa}= ) ( 17.54 times 0.99=17.36 mathrm{mm} mathrm{hg} ) hence , vapour pressure of water in the solution is ( 17.36 mathrm{mm} ) hg

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