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# Osmolarity Questions

The number of moles of solute per unit volume of solution is measured by molarity, while the number of osmoles of solute particles per unit volume of solution is measured by osmolarity. This value enables the measurement of a solution’s osmotic pressure and the determination of how the solvent will diffuse across a semipermeable membrane (osmosis) separating two solutions with different osmotic concentrations.

 Definition: Osmolarity is a concentration expression that reflects the osmoticity of solutions. It is an osmolar concentration expression. The number of solute particles per 1 L of solvent is referred to as osmolarity. A solution’s osmolarity is typically expressed as Osm/L.

## Osmolarity Chemistry Questions with Solutions

Q1. The osmolarity of a solution containing a 1M NaCl solution is ____.

a.) 1

b.) 2

c.) 3

d.) None of the above

Correct Answer– (b.) 2.

Q2. Osmolarity is independent of ____.

a.) number of particles

b.) moles

c.) nature of particles

d.) All of the above

Correct Answer– (c.) nature of particles.

Q3. The osmolarity of a solution containing a 1M CaCl2 solution is ____.

a.) 1

b.) 2

c.) 3

d.) None of the above

Correct Answer– (c.) 3.

Q4. Osmolarity is the number of osmoles ____.

a.) per litre of solvent

b.) per kg of solvent

c.) per litre of solution

d.) per kg of solution

Correct Answer– (c.) per litre of solution.

Q5. A solution with high osmolarity has ____ water molecules with respect to solute particles.

a.) larger

b.) equal

c.) fewer

d.) None of the above

Correct Answer– (c.) fewer.

Q6. Define osmolarity.

Answer. Osmolarity is defined as the number of particles or the concentration of a specific solute or solutes per litre of solvent. Osmoles are used to measure osmolarity.

Q7. If 4 moles of MgCl2 are dissolved in 2 L of water. What will the osmolarity be?

Answer. 1 mole of MgCl2 gives 3 pieces = 3 osmoles = 12.

Therefore, the Osmolarity of 4 moles of MgCl2 = 12 × ½ = 6 OsM.

Q8. How will you calculate osmolarity if there are many solutes in the solution?

Answer. If a solution contains many different types of solutes (for example, both glucose and sodium chloride), its osmolarity can be calculated using the equation

Osmolarity = Sum of all (molarity × n) of all solutes

n: the number of particles released from the solute molecule.

Q9. What is the osmolarity of a 2.6 M solution of table salt?

Answer. NaCl dissociates completely in water to form Na+ ions and Cl ions.

Thus, each mole of NaCl becomes two osmoles in the solution: one mole of Na+ and one mole of Cl.

Therefore, osmolarity = 2 × 2.6 M = 5.2 osmoles.

Q10. How can osmolarity be determined?

Answer. To determine the osmolarity of a solution, first, determine the number of osmoles of solute per litre of solvent. An osmole is a unit of measurement for the number of moles in a compound. A Mole is a unit of measurement for the amount of matter in a substance that contains precisely 6.02214086 × 1023 particles. The Avogadro constant is another name for this number. The following is a simplified formula for osmolarity. If there is more than one solute, the osmolarity is the sum of the solute molarities per litre of solvent.

Osmolarity (osmol) = Osmoles/litre

Q11. Differentiate between osmolarity and osmolality.

Answer. Osmolarity is defined as the concentration in moles per litre of solution, whereas osmolarity is defined as the concentration in moles per kilogram of solvent. However, for dilute solutions, they are numerically almost the same.

Since the volume of the solvent varies with temperature, osmolarity measurements are temperature dependent (i.e., the volume is larger at higher temperatures). In contrast, osmolality, which is based on mass of the solvent is temperature independent.

Q12. What is the osmolarity of a glass of skim milk? (There are 13 g of lactose in 236 mL)

Answer. Lactose is a kind of sugar with a molecular weight of about 342 g per mole.

13 g lactose × 1 mole/342 g = 0.038 mole.

Osmolarity = 0.038 mole / 0.236 litre = 0.16 OsM

Q13. A solution contains 312 mg of K+ ions per 200mL. How many milliosmoles are represented in a litre of the solution?

Answer. Grams of potassium ions present in 1000 mL

Conversion of mg to g:

= 312 mg × 1g/1000mg = 0.312g

$$\begin{array}{l}Grams\ of\ potassium\ ions\ present\ in\ 1L =\frac{0.312 g of K ions}{200 mL}=\frac{x}{1000 mL}=1.56 g\end{array}$$
$$\begin{array}{l}milliosmoles\ per\ litre=\frac{mass\ in\ g}{molecular\ weight}\times number\ of\ particles\times 1000\end{array}$$

Mass in g = 1.56 g

Molecular weight of potassium = 39

Number of particles = 1 as only potassium ions are present in the solution.

$$\begin{array}{l}milliosmoles\ per\ litre=\frac{1.56}{39}\times 1\times 1000=40\end{array}$$

Hence, 40 milliosmoles of potassium are represented in a litre of the solution.

Q14. Calculate the osmolarity of 0.2 M of Na2SO4.

Answer. Given, M = 0.2 M

Osmolarity = Molarity × n

Na2SO4 dissociates into 2Na+ + SO42–

Therefore, n = 3

Hence, osmolarity will be 0.2 × 3 = 0.6 OsM

Q15. A solution contains 1% of anhydrous dextrose in water forinjection. How many milliosmoles per litre are represented by this concentration? (m.w = 180)

Answer. 1% of anhydrous dextrose = 1g/100 mL = x/1000 mL = 10 g

$$\begin{array}{l}Milliosmoles\ per\ litre =\frac{mass\ in\ g}{molecular\ weight}\times number\ of\ particles\times 1000 \end{array}$$

Mass in g = 10 g

Molecular weight of dextrose = 180

Number of particles = 1 as dextrose is a non electrolyte

$$\begin{array}{l}milliosmoles\ per\ litre =\frac{10\ g}{180}\times 1\times 1000 =56\end{array}$$

## Practise Questions on Osmolarity

Q1. The osmolarity of a solution containing 1M sucrose solution is ____.

a.) 1

b.) 2

c.) 3

d.) None of the above

Q2. For dilute solutions, the value of osmolarity is ___ osmolality.

a.) greater than

b.) similar to

c.) smaller than

d.) None of the above

Q3. What is the approximate osmolarity of the sugar in a can of Red Bull? (There are 27 g of sugar in a 250 mL can).

Q4. What is the osmolarity of seawater given the following:

Solute

g / L

Molecular weight

Cl

19

35

Na+

10.5

23

Mg

1.3

24

S

0.8

32

Q5. Can osmolarity be calculated from osmolality? If yes, explain how?

Click the PDF to check the answers for Practice Questions.