Question

Calculate the amount of water (in grams) that must be added to each of the following substances to produce the indicated solution.

a) 3.50 g of urea(NH2)2CO in the preparation of a 22.5 percent by mass solution.
b) 30.2 g of MgCl2 in the preparation of a 5.6 percent mass solution.

Answer 1

a) To prepare a 22.5% by mass solution of urea, we need to add enough water to dissolve 3.50 g of urea and make a total mass of 100 g (since the mass percentage is calculated based on the total mass of the solution). Let's call the mass of water we need to add x.

The mass of urea will be 3.50 g, and the mass of the resulting solution will be 100 g, so the mass of water we need to add is:

x = 100 g - 3.50 g = 96.50 g

Therefore, we need to add 96.50 g of water to 3.50 g of urea to prepare a 22.5% by mass solution.

b) To prepare a 5.6% by mass solution of MgCl2, we need to add enough water to dissolve 30.2 g of MgCl2 and make a total mass of 100 g. Let's call the mass of water we need to add x.

The mass of MgCl2 will be 30.2 g, and the mass of the resulting solution will be 100 g, so the mass of water we need to add is:

x = 100 g - 30.2 g = 69.8 g

Therefore, we need to add 69.8 g of water to 30.2 g of MgCl2 to prepare a 5.6% by mass solution.