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

$\text{Osmolality} = (\text{number of ions}) \times m$

Question 1

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

Accepted Answer

Molality is defined as the number of moles of solute per kilogram of solvent: $m = \frac{	ext{mol solute}}{	ext{kg solvent}}$.

Question 2

A solution is prepared by dissolving 24.8 g of sucrose ($\mathrm{C_{12}H_{22}O_{11}}$) in 550.0 g of water. What is the molality of the solution?

Accepted Answer

$0.132~m$

Question 3

Which property makes molality ($m$) preferable over molarity ($M$) when studying colligative properties?

Accepted Answer

Molality is not affected by changes in temperature or pressure.

Question 4

A solution is prepared by dissolving 43.0 g of potassium chlorate ($\mathrm{KClO_3}$) in enough water to make 100.0 mL of solution. If the density of the solution is $1.76~\mathrm{g/mL}$, what is the molality of $\mathrm{KClO_3}$ in the solution?

Accepted Answer

$2.64~m$

Question 5

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

Accepted Answer

$31.2~m$

Question 6

Which formula correctly expresses osmolality for an ionic compound?

Accepted Answer

$	ext{Osmolality} = (	ext{number of ions}) 	imes m$

Question 7

What is the osmolality of total ions in an aqueous solution prepared by dissolving $0.400$ moles of $\mathrm{Pb(NO_3)_2}$ in $750.0~\mathrm{g}$ of water?

Accepted Answer

$2.67~m$

Question 8

A solution contains $2.5~m$ NaCl. What is the osmolality of the solution?

Accepted Answer

$5.0~m$

Question 9

What is the ionic molality of sodium ions in a solution of $25.7~g$ NaNO$_3$ dissolved in enough water to make a $150.0~mL$ solution? Density of the solution is $1.02~\mathrm{g/mL}$.

Accepted Answer

$2.57~m$

Question 10

Which of the following best describes the difference between molality and osmolality?

Accepted Answer

Molality measures moles of solute per kilogram of solvent, while osmolality measures the total number of dissolved particles per kilogram of solvent.

Question 11

A hospital prepares a saline solution by dissolving $58.5~g$ NaCl in $1.00~kg$ of water. What is the osmolality of the solution?

Accepted Answer

$2.0~m$

Question 12

If a solution contains $0.250~m$ $\mathrm{CaCl_2}$, what is the osmolality of the solution?

Accepted Answer

$0.750~m$

Question 13

A chemist needs to prepare a solution with a molality of $2.00~m$ using $0.500~kg$ of solvent. How many moles of solute are required?

Accepted Answer

$1.00~	ext{mol}$

Question 14

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

Accepted Answer

Dissolving a compound that dissociates into more ions.

Question 15

A solution is made by dissolving $0.100~	ext{mol}$ of glucose ($\mathrm{C_6H_{12}O_6}$) in $0.500~	ext{kg}$ of water. What is the osmolality of the solution?

Accepted Answer

$0.200~m$

Question 16

Which of the following solutions will have the highest osmolality, assuming equal molality?

Accepted Answer

$\mathrm{Na_2SO_4}$

Molality and Osmolality: Concentration Units in Solution Chemistry

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