A urea solution weighing 500 g has a solute mole fraction of 0.2. Water is added to dilute it to a total mass of 1500 g. What is the mole fraction of urea in the diluted solution?

0.0625
0.0667
0.1000
0.2000

Correct answer: 0.0625

Solution

Solving the 500 g solution gives about 5 mol urea and 20 mol water; adding 1000 g (55.5 mol) water makes the new mole fraction 5/(5+75.5) approx 0.0625.

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