Question

determine the vapor pressure of a solution at 55 celsius that contains 34.2g NaCl in 375 mL of water. the vapor pressure of pure water at 55 celsius is 118.1 torr. the van't hoff factor for NaCL is 1.9

Answer 1

raoults law = in a soln the vapour pressure of a component at a given temperature is equal to mole fraction of that component multiplied by vapour pressure of that component in pure state .

Pa = XaPao

but NaCl is a stron electrolyte so it will be complete dissociated ...
no of moles of NaCl = 34.2 / 58.5 = 0.59 (molecular mass of NaCl = 58.5 g/mole)

since NaCl is completely dissociated into Na+ and Cl- ...

NaCl ------> Na+ + Cl-

1 mole NaCl gives 1 mole of Na+ and 1 molee of Cl-

so number of moles of Na+ = 0.59
number of moles of Cl- = 0.59

number of moles of water = 375/18 = 20.83 (molar mass of water = 18 g/mole)

so total number of moles of all species in soln = 20.83 + 0.59 + 0.59 = 22.01

mole fraction of water = number of moles of water / total number of moles = 20.83/22.01 = 0.9463

so vapour pressure of NaCl soln . = 0.9463 X 118.1 = 111.7684 torr