Question

Be sure to answer all parts. Calculate the amount of water (in grams) that must be added to each of the following substances to produce the indicated solution. (a) 7.70 g of urea (NH2)2CO in the preparation of a 20.1 percent by mass solution: (b) 22.4 g of MgCl2 in the preparation of a 6.6 percent mass solution:

Answer 1

a) Mass percent = ( mass of urea / mass of solution ) x 100

20.1 / 100 = 7.70 g / mass of solution

0.201 = 7.70 / mass of solution

mass of solution = 7.70 / .201

= 38.31g

Mass of water = mass of solution - mass of urea

= 38.31 - 7.70 = 30.61 g

Mass of water added = 30.61 g

b) Mass percent = ( mass of MgCl₂ / mass of solution ) x 100

6.6 = ( 22.4 / mass of solution ) x 100

6.6/100 = 22.4 / mass of solution

.066 = 22.4 / mass of solution

mass of solution = 22.4 / .066

= 339.39 g

mass of water = mass of solution - mass of MgCl₂

= 339.39 - 22.4

= 316.99 g

mass of water added = 316.99 g