Question

Calculate the vapor pressure for water when adding 10g of glucose to 1L of water at 20 degrees celcius. (The vapor pressure of water at this temperature is 17.5 torr)

Answer 1

Calculate the vapor pressure for water when adding 10g of glucose to 1L of water at 20 degrees celcius. (The vapor pressure of water at this temperature is 17.5 torr)

Solution:- density of water = 1 g/ml

thus, mass of 1 litres of water = 1000 g

molar mass of water = 18 g/mole

thus, moles of 1000 g of water = 1000/18 = 55.56 mole

Now, molar mass of glucose = 180 g/mole

thus, moles of 10 g of glucose = 10/180 = 0.0556 moles

Mole fraction of glucose = moles of glucose/(moles of glucose + moles of water) = 0.0556/(55.56+0.0556) = 0.001

Now, As per Raoult's Law, fractional change in vapour pressure = mole fraction of solute

Let the vapour pressure of the glucose-water solution be P

Thus, (17.5-P)/17.5 = 0.001

or, P = 17.4825 torr