Question

In: Chemistry

6. Sometimes it's hard to tell which thing is the solvent and which the solute –...

6. Sometimes it's hard to tell which thing is the solvent and which the solute – for example you can make mixtures of water and methanol at any proportion. Fortunately, Raoult's law doesn't care which thing you call the solvent or solute; at a given temperature, both things have vapor pressures that fit the equation    PA = ΧA PA0 - where the right-hand side of the equation is the mole fraction times the pure liquid vapor pressure at that temperature.

a.) Calculate the vapor pressures for methanol (CH3OH, 32.04 g/mol, P0 = 13.02 kPa) and water (18.02 g/mol, P0 = 3.20 kPa) above a mixture containing one mole each of methanol and water (in a sealed container with a head-space for the vapor).

b.) Which of the two components' (methanol's or water's) vapor pressure would be depressed the most by addition of 10 g of NaCl (58.44 g/mol) assuming complete dissolution by a mechanism similar to that which takes place in aqueous solution (similar IMFs)? Calculate the percent change in that component's vapor pressure.

Solutions

Expert Solution

moles of methanol = moles of water = 1

total moles = 2

mole fraction of each = 0.5

vapour pressure of solution = Pm Xm + PwXw

                                              = 13.02 x 0.5 + 3.20 x 0.5

                                              = 8.11 kPa

vapour pressure of solution    = 8.11 kPa

2)

here in methanol NaCl does not dissolve .

so NaCl i value = 1

in water will give 2 ions .

so NaCl i value = 2 .

vapour pressure is directly proportional to i value.

NaCl i value is more in water. in that vapour pressure is most depressed.


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