Which of the following pairs of solutions is NOT isotonic (does not have equal osmotic pressure) at the same temperature?

0.2 M urea and 0.1 M NaCl
0.3 M urea and 0.1 M MgCl2
0.1 M Na2SO4 and 0.1 M NaCl
0.1 M Na2SO4 and 0.1 M Ca(NO3)2

Correct answer: 0.1 M Na2SO4 and 0.1 M NaCl

Solution

Effective concentrations (i*M): A) 0.2*1=0.2 vs 0.1*2=0.2: ISOTONIC. B) 0.3*1=0.3 vs 0.1*3=0.3: ISOTONIC. C) 0.1*3=0.3 (Na2SO4) vs 0.1*2=0.2 (NaCl): NOT equal, NOT ISOTONIC. D) 0.1*3=0.3 (Na2SO4) vs 0.1*3=0.3 (Ca(NO3)2): ISOTONIC. So pair C is NOT isotonic.

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