QUESTION 22 If ethanol (boiling point, 78ºC) and methanol (boiling point, 65ºC)

Question

QUESTION 22

If ethanol (boiling point, 78ºC) and methanol (boiling point, 65ºC) are placed in side-by-side beakers in an enclosed chamber, at room temperature, what will be the partial pressures of the air inside the chamber when equilibrium is reached?

more ethanol vapor pressure than methanol

equal vapor pressures of ethanol and methanol

less ethanol vapor pressure than methanol

3 points   

QUESTION 23

If ethanol (boiling point 78ºC) and methanol (boiling point 65ºC) are mixed together in equimolar amounts and placed in an enclosed chamber at room temperature, what will be the partial pressures of the air inside the chamber when equilibrium is reached?

more ethanol vapor pressure than methanol

equal vapor pressures of ethanol and methanol

less ethanol vapor pressure than methanol

3 points   

QUESTION 24

A mixture of 2% ethanol in water can be distilled to produce vapor that is 10% (or more) ethanol in water, and this vapor can then be condensed to give a higher alcoholic content than the original liquid mixture. What law explains this?

the law of conservation of matter

Hess’s law

Raoult’s law

the ideal gas law

3 points   

QUESTION 25

Three nonpolar, volatile substances are present in equimolar amounts in a solution inside a closed container. The gas above the solution is found to contain much more of component A than of B, and considerably more of B than C. What, if anything, can be deduced about the boiling points of A, B, and C?

Nothing whatever can be concluded about their boiling points.

A is highest-boiling, B is next, and C is lowest.

C is highest-boiling, B is next, and A is lowest.

They have comparable boiling points.

more ethanol vapor pressure than methanol

equal vapor pressures of ethanol and methanol

less ethanol vapor pressure than methanol

Explanation / Answer

Question 22: Answer is c). Less ethanol vapor pressure than methanol.

Ethanol's vapor pressure at room temperature is 5.95 kPa, while methanol's vapor pressure at the same temperature is 13.02 kPa.

Because methanol has a lower molecular weight when compared with ethanol, which indicates it has weaker intermolecular forces.

Both methanol (CH3OH) and ethanol (CH3CH2OH) have the capacity to form hydrogen bonds, so from this viewpoint there is no difference.

However, the difference in London dispersion forces.

Ethanol has a molecular weight of 46 amu, while methanol has a molecular weight of 32 amu. This difference in molecular weight means stronger London dispersion forces in favor of the heavier molecule (ethanol) which in turn translates into a lower vapor pressure. Thus, the molecule that shows weaker intermolecular forces (methanol) will have the higher vapor pressure.

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