Calculate the amount of water (in grams) that must be added to (a) 5.00 g of urea (NH2) 2CO in the preparation of a 16.2 percent by mass solution, and (b) 26.2 g of MgCl2 in the preparation of a 1.5 percent by mass solution.

a. 25.8 g of water

b. 1720.4 g of water

Explanation:

A percent by mass, means the grams of solute in 100 g. of solution. So, 16.2 g of urea are contained in 100 g of solution.

Then, solute mass + solvent mass = solution mass

16.2 g of urea + solvent mass = 100 g

solvent mass = 100 g - 16.2 g → 83.8 g

Now we can make the rule of three:

16.2 g of urea use 83.8 g of water

then, 5 g of urea would use (5. 83.8) / 16.2 = 25.8 g of water

b. 1.5 % by mass, means 1.5 g of solute in 100 g of solution.

So water mass, for this solution will be 100 g - 1.5 g = 98.5 g

Now, we apply the rule of three:

1.5 g of solute use 98.5 of water

26.2 g of solute will use (26.2. 98.5) / 1.5 = 1720.4 g