Osmotic pressure: Class 12 practice
π = CRT = (n/V)RT. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Molarity
The idea behind osmotic pressure
π = CRT = (n/V)RT. Measuring π gives the molar mass of a solute: M = wRT/(πV). Use R = 0.0821 L atm mol⁻¹ K⁻¹, V in litres and T in K.
Step-by-step method
- Convert volume to L and temperature to K.
- π = (w/M)RT/V.
- Rearrange for the unknown.
Worked example 1 (Basic)
6 g of a solute (M = 342 g/mol) is dissolved to make 100 mL at 285 K. Find the osmotic pressure.
- C = 6/342/0.1
- π = CRT = 4.105 atm
- Answer: 4.105 atm
Worked example 2 (Standard)
13 g of a solute (M = 20000 g/mol) is dissolved to make 1000 mL at 286 K. Find the osmotic pressure.
- C = 13/20000/1
- π = CRT = 0.01526 atm
- Answer: 0.01526 atm
Worked example 3 (Challenge)
19 g of a protein in 500 mL gives π = 5.477 atm at 316 K. Find its molar mass.
- M = wRT/(πV)
- Answer: 180 g/mol
Common mistakes and misconceptions
- Using mL
- Using °C
Practice questions
- (Basic) 4 g of a solute (M = 60 g/mol) is dissolved to make 250 mL at 300 K. Find the osmotic pressure.
- (Basic) 16 g of a solute (M = 342 g/mol) is dissolved to make 500 mL at 280 K. Find the osmotic pressure.
- (Standard) 18 g of a solute (M = 60 g/mol) is dissolved to make 250 mL at 309 K. Find the osmotic pressure.
- (Standard) 2 g of a solute (M = 5000 g/mol) is dissolved to make 250 mL at 316 K. Find the osmotic pressure.
- (Challenge) 20 g of a protein in 250 mL gives π = 10.40 atm at 285 K. Find its molar mass.
- (Challenge) 11 g of a protein in 100 mL gives π = 0.5202 atm at 288 K. Find its molar mass.
Answer key with full working
- Q1: C = 4/60/0.25 → π = CRT = 6.568 atm → Answer: 6.568 atm
- Q2: C = 16/342/0.5 → π = CRT = 2.151 atm → Answer: 2.151 atm
- Q3: C = 18/60/0.25 → π = CRT = 30.44 atm → Answer: 30.44 atm
- Q4: C = 2/5000/0.25 → π = CRT = 0.04151 atm → Answer: 0.04151 atm
- Q5: M = wRT/(πV) → Answer: 180 g/mol
- Q6: M = wRT/(πV) → Answer: 5000 g/mol
Syllabus note
This skill is listed under "Solutions" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is osmotic pressure in?
"Solutions", Class 12 Chemistry (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.