13.1.3 Molality

Question 1

Molality

Answer 1

• Molality is a unit of concentration that does not depend on temperature.
• The density and molarity of a solution together provide enough information to find the molality of a solution.

Question 2

note

Answer 2

• Molality does not depend on the volume of a solution, so it is often useful in situations where the volume or temperature changes. One molal (m) is one mole of solute per kilogram of solvent.
• Problem: 22.4 g of MgCl 2 is completely dissolved in 2.00 L H 2 O. If the density of water is 1.00 g/mL and the density of the final salt solution is 1.089 g/mL, what is the molality of the MgCl 2 solution?
• First, the mass of solvent must be found. Since mass equals volume times density, the mass of water (the solvent) is 0.200 kg.
• Next, the molar mass of the solute is used to convert grams of solute to moles.
• The molality (1.18 m) and the molarity (1.15 M) of this
  solution are very close in value. Typically, molarity and
  molality values of aqueous solutions are within 10% of each other.
• The density and molarity of a solution together provide enough information to find the molality of a solution.
• Problem: The density of a 2.45 M aqueous solution of methanol (CH 3 OH) is 0.976 g/mL. What is the molality of the methanol solution?
• First, the mass of one liter of solution can be found given the density ( ) of the solution.
• Since this is the total mass of the solution, the mass of methanol (the solute) must be subtracted in order to obtain the mass of water (the solvent). The mass of methanol is found by converting molarity of methanol to moles of methanol, and then converting moles of methanol to grams using molar mass.
• Finally, moles of methanol are divided by the mass of water in kilograms to obtain the molality of the solution.

Question 3

What is the molality of a battery acid solution that has a molarity of 4.19 M? The density of the solution is 1.25 g / mL. The molar mass of the solute is 98.08 g / mol.

Answer 3

4.99 mol / kg

Question 4

The density of a 1.83 M aqueous solution of methanol is 0.976 g / mL. The molar mass of the solute is 32.04 g / mol. What is the molality of the solution?

Answer 4

2.00 mol / kg

Question 5

Which of the following statements about molarity or molality is not true?

Answer 5

Molarity is sometimes more helpful because it is independent of volume and not sensitive to changes in temperature.

Question 6

A solution has a molarity of 1.56 M and a density of 1.38 g / mL. The solute’s molar mass is 289.7 g / mol. What is the molality of the solution? Assume the density of the water is 1.00 g / mL.

Answer 6

1.68 mol / kg

Question 7

A solution is made by dissolving 33.7 g of MgCl2 and in 0.35 L of H2O. What are the mass of the solvent and the number of moles of the solute, respectively, in the solution? The density of water is 1.00 g / mL.

Answer 7

0.35 kg H2O; 0.354 mol MgCl2

Question 8

A battery acid solution has a molarity of 3.83 M, a molar mass of 98.08 g / mol and a density of 1.25 g / mL. What is the mass of the solvent in one liter of solution?

Answer 8

0.874 kg

Question 9

What mass of potassium nitrate (KNO3 ) should be added to 5.00 × 10^2 g of H2O in order to make a 0.300 m KNO3 solution? The molar mass of KNO3 is 101.1 g / mol

Answer 9

15.2 g

Question 10

Find the molality of an MgCl2 solution that has 19.8 g of solute in 0.65 L of H2O. The molar mass of MgCl2 = 95.22 g / mol MgCl2. Assume that the density of water is 1.00 g / mL.

Answer 10

0.32 mol / kg

Question 11

Suppose that you have a solution of battery acid that has a molarity of 3.83 M. The molar mass of the solute is 98.08 g / mol. What is the mass of the solute in one liter of the battery acid? (Assume one liter is an exact number.)

Answer 11

0.376 kg

Question 12

Suppose that you have 0.534 moles of solute in 0.430 L of water. What is the molality of the solution? Assume that the density of water is 1.000 g / mL.

Answer 12

1.24 mol / kg