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Consider separate solutions of 0.500 M C2H5OH(aq), 0.100 M Mg3(PO4)2(aq), 0.250 M KBr(aq) and 0.125 M Na3PO4(aq) at 25 °C. Which statement is true about these solutions, assuming all salts to be strong electrolytes?

1. They all have the same osmotic pressure.
2. 0.100 M Mg3(PO4)2(aq) has the highest osmotic pressure.
3. 0.125 M Na3PO4(aq) has the highest osmotic pressure.
4. 0.500 M C2H5OH(aq) has the highest osmotic pressure.

Correct answer: They all have the same osmotic pressure.

Solution

Effective particle concentration i x M: ethanol 1x0.5=0.5; Mg3(PO4)2 5x0.1=0.5; KBr 2x0.25=0.5; Na3PO4 4x0.125=0.5. All equal, so all solutions have the same osmotic pressure (index 0).

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