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For solutions of equal molarity, which one would exert the greatest osmotic pressure?

1. 1 M NaCl
2. 1 M MgCl2
3. 1 M (NH4)3PO4
4. 1 M Na2SO4

Correct answer: 1 M (NH4)3PO4

Solution

Osmotic pressure pi = i*C*R*T. For equal molarity it scales with i: NaCl i=2, MgCl2 i=3, Na2SO4 i=3, (NH4)3PO4 i=4. (NH4)3PO4 gives the most ions (4), so it exerts the greatest osmotic pressure.

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