Q: Which of the following is a correct expression for calculating molality ($$m$$)?

$$m = \frac{\text{moles of solute}}{\text{kg of solvent}}$$

Question 1

Which of the following best defines molality ($$m) in $$chemistry?

Accepted Answer

The number of moles of solute per kilogram of solvent.

Question 2

A solution is prepared by dissolving 0.30 moles of NaCl in 1 kg of water. What is the molality of the solution?

Accepted Answer

$$0.30\ 	ext{mol/kg}$$

Question 3

If you dissolve 43 g of potassium nitrate ($$	ext{KNO}_3) in $$enough water to make 100.0 g of solution, and the density of the solution is $$1.790\ 	ext{g/mL}$$, what is the molality of $$	ext{KNO}_3 in $$the solution? (Molar mass of $$	ext{KNO}_3 = 101.1\ 	ext{g/mol})$$

Accepted Answer

$$0.430\ 	ext{mol/kg}$$

Question 4

Which of the following is a correct expression for calculating molality ($$m$$)?

Accepted Answer

$$m = \frac{	ext{moles of solute}}{	ext{kg of solvent}}$$

Question 5

A chemist prepares an aqueous solution by dissolving $$10.0\ 	ext{g} of $$glucose ($$	ext{C}_6	ext{H}_{12}	ext{O}_6) in$$ $$250\ 	ext{g} of $$water. What is the molality of the solution? (Molar mass of glucose = $$180\ 	ext{g/mol})$$

Accepted Answer

$$0.222\ 	ext{mol/kg}$$

Question 6

Why is molality preferred over molarity when studying colligative properties such as boiling point elevation?

Accepted Answer

Because molality depends on the mass of solvent, which does not change with temperature.

Question 7

A solution contains $$5.00\ 	ext{g} of $$NaOH ($$	ext{M}_	ext{NaOH} = 40.0\ 	ext{g/mol}) $$dissolved in $$200.0\ 	ext{g} of $$water. What is the molality of the solution?

Accepted Answer

$$0.625\ 	ext{mol/kg}$$

Question 8

Which of the following statements is TRUE regarding the calculation of molality?

Accepted Answer

Molality is based on the mass of solvent, not the total solution.

Question 9

A laboratory technician needs to prepare a $$2.00\ 	ext{mol/kg}$$ solution of $$	ext{KCl}. $$How many grams of $$	ext{KCl} ($$molar mass = $$74.6\ 	ext{g/mol}) $$are needed to dissolve in $$500\ 	ext{g} of $$water?

Accepted Answer

$$149.2\ 	ext{g}$$

Question 10

A solution is made by dissolving $$12.0\ 	ext{g} of $$NaCl in $$250\ 	ext{g} of $$water. What is the molality of the solution? (Molar mass of NaCl = $$58.5\ 	ext{g/mol})$$

Accepted Answer

$$0.410\ 	ext{mol/kg}$$

Question 11

If the density of a solution is $$1.25\ 	ext{g/mL}$$ and the solution contains $$25.0\ 	ext{g} of $$solute in $$200.0\ 	ext{g} of $$solvent, what is the molality if the molar mass of the solute is $$50.0\ 	ext{g/mol}$$?

Accepted Answer

$$2.50\ 	ext{mol/kg}$$

Question 12

A solution is prepared by dissolving $$20.0\ 	ext{g} of $$urea ($$	ext{CO(NH}_2)_2$$, molar mass = $$60.0\ 	ext{g/mol}) in$$ $$500.0\ 	ext{g} of $$water. What is the molality of the solution?

Accepted Answer

$$0.400\ 	ext{mol/kg}$$

Question 13

Which of the following would NOT affect the molality of a solution?

Accepted Answer

Changing the temperature.

Question 14

A solution is made by dissolving $$5.85\ 	ext{g} of $$NaCl in $$100.0\ 	ext{g} of $$water. What is the molality of the solution? (Molar mass of NaCl = $$58.5\ 	ext{g/mol})$$

Accepted Answer

$$1.00\ 	ext{mol/kg}$$

Question 15

A chemist needs to prepare $$250\ 	ext{g} of a$$ $$0.500\ 	ext{mol/kg}$$ solution of glucose ($$	ext{C}_6	ext{H}_{12}	ext{O}_6$$, molar mass = $$180\ 	ext{g/mol}). $$How many grams of glucose are required?

Accepted Answer

$$22.5\ 	ext{g}$$

Study Notes on Molality and Solution Concentration

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