0.5 g of nonvolatile solute is dissolved in 100 g of ethyl acetate in 20C.the vapour pressure of the solution and pure ethyl acetate are 72 and 72.8 torr rezpectively at 20C. Calculate the molecular weight of the solute

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Please find below the solution to the query posted by you.

We are given,

Mass of non-volatile solute, m1 = 0.5 g
Mass of solvent (ethyl acetate, CH‚Äč3COOCH2CH3), m2 = 100 g
Molar mass of solvent, M2 = 88 g
T = 20 oC = 293 K
Vapour pressure of pure ethyl acetate, p2o = 72.8 torr
Vapour pressure of solution, ps = 72 torr
Mole fraction of the solute can be determined by using the formula for relative lowering of vapour pressurep2o - psp2o=m1 × M2M1 × m272.8 - 7272.8 = 0.5 × 88M1 × 100M1 = 0.5 × 88 × 72.80.8 × 100= 40.04 g mol-1.

Hence, the molar mass of the non-volatile solute is 40.04 g mol−1.

Hope this information will clear your doubt regarding relative lowering of vapour pressure.

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  • 5
relative lowering of vapour pressure
72.8-72/72.8 = mole fraction
 
thus 0 .8/72.8 = 0.5 * 88 / m* 100     ( as mm of ethyl acetate = 88 ad .5 <<100)
thus m = 12.54
  • 1
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