  Calculate the mass of propylene glycol that must be added to 0.340kg of water to reduce the vapor pressure by 2.88 torr at 40 degrees celsius.

Question

Calculate the mass of propylene glycol that must be added to 0.340kg of water to reduce the vapor pressure by 2.88 torr at 40 degrees celsius.

Step 1

Let us first calculate moles of water.

moles = mass/molar mass

given, mass of water = 0.340 kg = 340 g

molar mass of water= 18 g/mol

so, moles of water = 340 g/18 g/mol = 19 mol

Step 2

Using formula,

vapor pressure of solution = mole fraction of solvent x vapor pressure of pure solvent

We know, vapor pressure of water (at 40oC) = 55.3 torr (literature)

So, vapor pressure of solution = (55.3 - 2.88) torr = 52.42 torr

Step 3

mole fraction of solvent = moles of water/(moles of water + moles of propylene glycol)

Feeding vapor pressure of solution and vapor pressure of pure solvent in the above equation

mole fraction of solvent = vapor pressure of solution/vapor pressure of pure solvent

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