Question

Calculate the amount of water (in grams) that must be added to (a) 9.00 g of urea [(NH2)2CO] in the preparation of a 8.10 percent by mass solution: (b) 28.9 g of MgBr2 in the preparation of a 2.90 percent mass solution:

Answer 1

Percent by mass solution = (mass of the solute x 100)/ mass of the solution

a.

Given,

Mass of urea = 9.00 g

Percent by mass solution = 8.10

Let's say that the amount of water that must be added = X g

Therefore,

8.10 = (9.00 x 100)/ (9.00 + X)

or, 72.9 + 8.1X = 900

or, 8.1X = 827.1

or, X = 102.11 g

Therefore, the amount of water that must be added = 102.11 g

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b.

Given,

Mass of MgBr₂ = 28.9 g

Percent by mass solution = 2.90

Let's say that the amount of water that must be added = X g

Therefore,

2.90 = (28.9 x 100)/ (28.9 + X)

or, 83.81 + 2.9X = 2890

or, 2.9X = 2806.19

or, X = 968 g

Therefore, the amount of water that must be added = 967.65 g