13.3.3 Boiling Point Elevation Problem

Question 1

Boiling Point Elevation Problem

Answer 1

• Combustion analysis can be used to identify the ratios of elements in an unknown sample.
• Boiling point elevation can provide information useful in identifying an unknown substance.

Question 2

note

Answer 2

• Combustion analysis can be used to identify the ratios of elements in an unknown sample.
• In the example, the ratios by weight of the amounts of carbon and hydrogen in the unknown are divided by their atomic weight to obtain the molar mass ratios. The molar mass ratio is simplified to whole numbers giving the empirical formula for the compound.
• Boiling point elevation can provide information useful in identifying an unknown substance. Boiling point elevation can be used to calculate the molecular weight of the unknown. Combining the molecular weight with the empirical formula gives the molecular formula for the unknown.
• In the example, the relationship between concentration and boiling point elevation is used to calculate the number of moles of solute. The number of moles of solute is used with the known weight of solute (5.00 g) to calculate the molecular weight of the solute. The molecular weight is found to be twice the molecular weight of the empirical formula, leading to the conclusion that the unknown compound is naphthalene
  (C10H4)

Question 3

A mass of 0.627 g of an unknown hydrocarbon is dissolved in 100.0 g of benzene (C6H6 ). The solution boils at 80.23°C. The normal boiling point of benzene is 80.10°C. What is the molecular formula of the unknown compound?

Kb for benzene is 2.64°C kg / mol.

Answer 3

C10H8

Question 4

What is the molality of the solution made by dissolving 5.0 grams of sugar (C12H22O11 ) in 125 g of water?

Answer 4

0.12 moles / kg

Question 5

How much is the boiling point of water elevated by a 2.5 m solution of C6H12O6?

Answer 5

1.3°C

Question 6

When a solute is dissolved in a solvent, the boiling point is raised and the freezing point is lowered. What is the practical significance of this information?

Answer 6

• It can be used to protect car engines from freezing in cold weather.
• It can be used to make ice cream in the summer.
• It can be used to determine the molecular mass of an unknown substance.

Question 7

A solute is added to a solvent, changing its colligative properties. Three of the colligative properties are listed. Which correctly identifies the direction of the change of each colligative property?

Answer 7

Vapor pressure: down; Boiling point: up; Freezing point: down.

Question 8

A solute is added to a solvent changing its colligative properties. Three of the colligative properties are listed. Which correctly identifies the direction of the change of each colligative property?

Answer 8

Vapor pressure: down
Boiling point: up
Freezing point: down

Question 9

Which of the following does not affect the boiling point elevation of solution?

Answer 9

molar mass of the solvent