Question

if at certain temperature the vapour pressure of pure water is 25mm hg and that of very dilute aqueous urea solution is 24.5mm hg, the molality of solution is...

Answer 1

Explanation:

Here is the solution. hope it's helpful

Answer 2

Molality of solution is 1.13 mol/kg

Explanation:

As the relative lowering of vapor pressure is directly proportional to the amount of dissolved solute.

The formula for relative lowering of vapor pressure will be,

$\frac{p^o-p_s}{p^o}=i\times x_2$

where,

$\frac{p^o-p_s}{p^o}$= relative lowering in vapor pressure

i = Van'T Hoff factor = 1 (for non electrolytes)

$x_2$ = mole fraction of solute

$\frac{25-24.5}{25}=1\times x_2$

$x_2=0.02$

Given : 0.02 moles of urea area are dissolved in 1 mole of solution

moles of solvent (water) = moles of solution - moles of solute = 1 -0.02 = 0.98

Mass of solvent =$moles\times {\text {Molar mass}}=0.98\times 18=17.64g$

$Molarity=\frac{n\times 1000}{W_s}$

where,

n = moles of solute

$W_s$ = weight of solvent in g  

$Molality=\frac{0.02\times 1000}{17.64g}=1.13mole/kg$

Therefore, the molality of solution is 1.13 mole/kg

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