Q: Which formula correctly expresses molality ($m$)?

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

Q: Which equation correctly expresses osmolality for an ionic compound?

$\text{Osmolality} = m \times n$

Question 1

Which of the following statements about molality ($m$) is correct?

Accepted Answer

Molality is defined as moles of solute per kilogram of solvent.

Question 2

A solution is prepared by dissolving 24.8 g of sucrose ($C_{12}H_{22}O_{11}$) in 550.0 g of water. What is the molality of the solution? (Molar mass of sucrose = 342 g/mol)

Accepted Answer

$0.144\ m$

Question 3

Which formula correctly expresses molality ($m$)?

Accepted Answer

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

Question 4

A solution is prepared by dissolving 43.0 g of potassium chlorate ($KClO_3$) in enough water to make 100.0 mL of solution. The density of the solution is $1.760\ g/mL$. What is the molality of $KClO_3$ in the solution? (Molar mass of $KClO_3 = 122.6\ g/mol$)

Accepted Answer

$3.55\ m$

Question 5

The density of a $15.7\ M$ methanol ($CH_3OH$) solution is $0.858\ g/mL$. If $H_2O$ is the solvent, what is the molality of the solution?

Accepted Answer

$18.3\ m$

Question 6

Which of the following is true about osmolality?

Accepted Answer

Osmolality measures the number of dissolved ions per kilogram of solvent.

Question 7

What is the osmolality of a solution prepared by dissolving $0.400$ moles of $Pb(NO_3)_2$ in $750.0\ g$ of water? ($Pb(NO_3)_2$ dissociates into 3 ions)

Accepted Answer

$1.60\ osmol/kg$

Question 8

Which equation correctly expresses osmolality for an ionic compound?

Accepted Answer

$	ext{Osmolality} = m 	imes n$

Question 9

A solution is made by dissolving $25.7\ g$ of $NaNO_3$ in enough water to make $150.0\ mL$ of solution. The density of the solution is $1.02\ g/mL$. What is the ionic molality of sodium ions in the solution? (Molar mass of $NaNO_3 = 85.0\ g/mol$)

Accepted Answer

$2.57\ m$

Question 10

If $2.5\ m$ $NaCl$ is dissolved in water, what is the osmolality of the solution?

Accepted Answer

$5.0\ osmol/kg$

Question 11

Which of the following best explains why molality is preferred over molarity in experiments involving temperature changes?

Accepted Answer

Molality is based on mass of solvent, which does not change with temperature.

Question 12

A nurse needs to prepare a solution with a specific osmolality for intravenous use. Why is it important to consider the number of ions produced by the solute?

Accepted Answer

Because osmolality depends on the total number of dissolved particles, not just the amount of solute.

Question 13

Which of the following would increase the osmolality of a solution the most?

Accepted Answer

Dissolving a salt that dissociates into three ions per formula unit.

Question 14

A patient receives an IV solution containing $0.400$ moles of $K_2SO_4$ in $750.0\ g$ of water. What is the osmolality of the solution? ($K_2SO_4$ dissociates into 3 ions)

Accepted Answer

$1.60\ osmol/kg$

Question 15

Which of the following is NOT a correct statement about osmolality?

Accepted Answer

Osmolality is measured in osmol/L of solution.

Question 16

A solution contains $0.250$ moles of $CaCl_2$ dissolved in $500.0\ g$ of water. What is the osmolality of the solution? ($CaCl_2$ dissociates into 3 ions)

Accepted Answer

$1.50\ osmol/kg$

Question 17

Which of the following would result in the lowest osmolality for a solution containing 1 mole of solute in 1 kg of solvent?

Accepted Answer

A covalent compound that does not dissociate.

Molality and Osmolality in Solution Chemistry: Definitions, Calculations, and Applications

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