Isotonic solution calculations (molecular weight method) MCQs With Answer

Introduction: Isotonic solution calculations (molecular weight method) are vital in pharmaceutics to ensure ophthalmic and parenteral formulations match the osmotic pressure of blood and tears. This method uses molecular weight, van’t Hoff factor (i), degree of dissociation (α), and osmolarity to compute isotonic concentrations. Key tools include the 0.9% NaCl reference (≈308 mOsm/L), sodium chloride equivalent (E value), and freezing point depression (isotonic ΔTf ≈ 0.52°C; 1% NaCl ΔTf ≈ 0.576°C). Mastery of formulas like Ciso = 0.0308 × (MW/i), E = (i_drug × MW_NaCl)/(i_NaCl × MW_drug), and adjustment steps prevents hemolysis or irritation. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. In the molecular weight method, isotonicity is achieved by matching which property to 0.9% NaCl?

• The number of osmotically active particles per liter
• The grams of solute per 100 mL
• The solution’s pH
• The solution’s viscosity

Correct Answer: The number of osmotically active particles per liter

Q2. What is the approximate osmolarity of 0.9% w/v sodium chloride solution at room temperature?

• 154 mOsm/L
• 308 mOsm/L
• 500 mOsm/L
• 900 mOsm/L

Correct Answer: 308 mOsm/L

Q3. Which formula correctly gives the isotonic concentration (% w/v) for a single solute using the molecular weight method?

• Ciso (% w/v) = 0.0308 × MW / i
• Ciso (% w/v) = 0.308 × MW × i
• Ciso (% w/v) = 0.9 × MW / i
• Ciso (% w/v) = 58.5 / (MW × i)

Correct Answer: Ciso (% w/v) = 0.0308 × MW / i

Q4. A nonelectrolyte with MW = 180 (i = 1). What % w/v makes it isotonic with blood?

• 1.90% w/v
• 5.55% w/v
• 0.90% w/v
• 10.0% w/v

Correct Answer: 5.55% w/v

Q5. Calculate the isotonic concentration (% w/v) of KCl (MW = 74.5; i ≈ 1.8).

• 0.90% w/v
• 1.27% w/v
• 1.90% w/v
• 2.60% w/v

Correct Answer: 1.27% w/v

Q6. Calculate the isotonic concentration of urea (MW = 60; i = 1).

• 0.90% w/v
• 1.85% w/v
• 5.55% w/v
• 3.08% w/v

Correct Answer: 1.85% w/v

Q7. Calculate the isotonic concentration of CaCl₂·2H₂O (MW = 147; i ≈ 2.6).

• 1.11% w/v
• 1.74% w/v
• 2.00% w/v
• 0.90% w/v

Correct Answer: 1.74% w/v

Q8. Which expression correctly relates van’t Hoff factor (i), degree of dissociation (α), and number of ions (n)?

• i = α × n
• i = 1 + α(n − 1)
• i = n − α
• i = (n − 1)/α

Correct Answer: i = 1 + α(n − 1)

Q9. Which set lists typical i-values at dilute concentrations?

• Non-electrolyte ≈ 1; 2-ion ≈ 1.8; 3-ion ≈ 2.6; 4-ion ≈ 3.4
• Non-electrolyte ≈ 0; 2-ion ≈ 2; 3-ion ≈ 3; 4-ion ≈ 4
• Non-electrolyte ≈ 1.8; 2-ion ≈ 1; 3-ion ≈ 2.6; 4-ion ≈ 3.4
• Non-electrolyte ≈ 1; 2-ion ≈ 2.6; 3-ion ≈ 1.8; 4-ion ≈ 3.4

Correct Answer: Non-electrolyte ≈ 1; 2-ion ≈ 1.8; 3-ion ≈ 2.6; 4-ion ≈ 3.4

Q10. Which formula for sodium chloride equivalent (E) is correct?

• E = (i_drug × MW_NaCl) / (i_NaCl × MW_drug)
• E = (i_NaCl × MW_drug) / (i_drug × MW_NaCl)
• E = MW_drug / MW_NaCl
• E = i_drug / i_NaCl

Correct Answer: E = (i_drug × MW_NaCl) / (i_NaCl × MW_drug)

Q11. What is the E-value of dextrose (MW = 180; i_drug = 1; i_NaCl ≈ 1.8; MW_NaCl = 58.5)?

• 0.10
• 0.18
• 0.36
• 0.52

Correct Answer: 0.18

Q12. Estimate the E-value of boric acid (MW = 61.8; i_drug = 1; i_NaCl ≈ 1.8; MW_NaCl = 58.5).

• 0.10
• 0.18
• 0.52
• 1.00

Correct Answer: 0.52

Q13. How much NaCl is required per 100 mL to make a 2% w/v solution of a drug (E = 0.18) isotonic?

• 0.18 g
• 0.36 g
• 0.54 g
• 0.90 g

Correct Answer: 0.54 g

Q14. The freezing point depression (ΔTf) of an isotonic solution with blood/tears is approximately:

• 0.20°C
• 0.52°C
• 0.576°C
• 1.00°C

Correct Answer: 0.52°C

Q15. The freezing point depression produced by a 1% w/v NaCl solution is approximately:

• 0.308°C
• 0.520°C
• 0.576°C
• 0.900°C

Correct Answer: 0.576°C

Q16. When using the molecular weight method, which osmolarity should you target for isotonicity with blood?

• 154 mOsm/L
• 250 mOsm/L
• 308 mOsm/L
• 600 mOsm/L

Correct Answer: 308 mOsm/L

Q17. How much NaCl must be added to 30 mL containing 1.0 g mannitol (E ≈ 0.18) to make it isotonic?

• 0.270 g
• 0.180 g
• 0.090 g
• No NaCl needed

Correct Answer: 0.090 g

Q18. Which compound can be isosmotic yet physiologically hypotonic due to membrane permeability, causing hemolysis?

• Sodium chloride
• Boric acid
• Urea
• Calcium chloride

Correct Answer: Urea

Q19. Hypertonic solutions relative to blood typically cause RBC:

• Hemolysis (swelling and rupture)
• Crenation (shrinking)
• No change
• Aggregation only

Correct Answer: Crenation (shrinking)

Q20. A drug X (MW = 250) dissociates into 2 ions with i = 1.5. What is its E-value (i_NaCl = 1.8; MW_NaCl = 58.5)?

• 0.150
• 0.172
• 0.195
• 0.308

Correct Answer: 0.195

Q21. What % w/v of boric acid (MW = 61.8; i = 1) is isotonic with tears by the molecular weight method?

• 0.90% w/v
• 1.50% w/v
• 1.90% w/v
• 2.60% w/v

Correct Answer: 1.90% w/v

Q22. If a solute forms 3 ions (n = 3) and i = 1.6 at a given concentration, what is the degree of dissociation (α)?

• 0.20
• 0.30
• 0.60
• 0.80

Correct Answer: 0.30

Q23. A monovalent electrolyte drug has MW = 340 and i = 1.8. What is its E-value (i_NaCl = 1.8; MW_NaCl = 58.5)?

• 0.10
• 0.172
• 0.25
• 0.52

Correct Answer: 0.172

Q24. How much NaCl per 100 mL is needed to make a 0.5% w/v solution of a non-electrolyte (MW = 150) isotonic? (i_NaCl = 1.8; MW_NaCl = 58.5)

• 0.108 g
• 0.792 g
• 0.900 g
• 0.500 g

Correct Answer: 0.792 g

Q25. What is the osmolarity of a 0.20 mol/L solution of anhydrous CaCl₂ with i ≈ 2.6?

• 200 mOsm/L
• 308 mOsm/L
• 520 mOsm/L
• 600 mOsm/L

Correct Answer: 520 mOsm/L

Q26. For predicting RBC response, which parameter is most relevant?

• Isosmoticity (all solutes)
• Tonicity based on non-penetrating solutes
• pH alone
• Viscosity

Correct Answer: Tonicity based on non-penetrating solutes

Q27. Which dosage forms most critically require isotonicity adjustment?

• Oral syrups only
• Topical ointments only
• Ophthalmic and intravenous solutions
• Suppositories only

Correct Answer: Ophthalmic and intravenous solutions

Q28. How many grams of NaCl are needed per liter to achieve isotonicity (≈308 mOsm/L)?

• 5.85 g
• 9.00 g
• 0.90 g
• 18.0 g

Correct Answer: 9.00 g

Q29. If α = 0.8 and a solute forms 4 ions (n = 4), what is the approximate i-value?

• 1.8
• 2.6
• 3.4
• 4.0

Correct Answer: 3.4

Q30. When only molecular weight and dissociation information are available (no ΔTf data), which tonicity method is most appropriate?

• Cryoscopic method
• Sodium chloride equivalent method
• Molecular weight method
• pH adjustment method

Correct Answer: Molecular weight method

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