Choose the substance with the lowest vapor pressure at a given temperature.

(a) bf3
(b) pf5
(c) becl2
(d) he
(e) co2

C

Explanation:

The vapor pressure is the pressure that the vapor does when it is in equilibrium with the liquid that originated it. So, it's a measure of the tendency of the boil of the liquid, and as higher is the vapor pressure, more easily will be to the liquid to boils, so lower will be the boiling point.

The boiling point depends on the strength of the intermolecular force of the substance and the molar mass of the substance. As higher is them, as higher is the boiling point.

BF3 is a nonpolar covalent compound, so it has London forces, which are the weakest. It has 67.82 g/mol of molar mass.

PF5 is also a nonpolar covalent compound and has London forces. Its molecular mass is 126 g/mol.

BeCl2 is an ionic compound formed by the ions Be+2 and Cl-, and the ionic force (ion-ion) is the strongest. Its molar mass is 80 g/mol.

He is a noble has, and so, has a weak force between its atoms. Its molar mass is 4 g/mol.

CO2 is a nonpolar compound, so it has London forces too. It has a molar mass of 44 g/mol.

So, the compound of the strong molar force is BeCl2, and at room temperature, it is solid (all ionic compounds are solid at room temperature). All the other compounds are gases at room temperature, so BeCl2 has the highest boiling point, and because of that, the lowest vapor pressure.

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