CHAPTER 4 PROBLEMS

PERCENT COMPOSITION


To calculate the percentage composition

v  Calculate the formula mass for the entire compound
v  Calculate the total mass for each element or elements for which the percentage is desired.
v  Divide the total mass of each element by the formula mass of the compound and multiply by 100.

1.  Determine the percentage of sodium in sodium sulfate, Na2SO4.

2.  For the compound sodium sulfate decahydrate, Na2SO4 •10H2O, calculate the following:

a. %Na b. %O c. %H2O

3.  Calculate the percentage of nitrogen in each of the following compounds:

a. NH4NO3 b. (NH4)2SO3 c. HNO2

4.  Calculate the mass of the metal in each of the following:

a.  50.0 grams MgS

b.  80.0 kilograms FeCO3

c.  25.0 kilograms PbCl2 • Pb(OH)2

d.  200.0 kilograms Al2O3

e.  150.0 grams SrCl2 • 2H2O

5.  Calcium chloride can exist as the anhydrous compound CaCl2 or in three different hydrated forms which are mono-, di-, and hexa- hydrates. Calculate the following:

a.  the percent calcium in each compound

b.  the percent water in the three hydrates

6.  The compound ammonia carbonate, (NH4)2CO3, when heated, decomposes completely to give ammonia, water, and carbon dioxide. Calculate (a) how many grams of ammonia and (b) how many grams of carbon dioxide will rust from the complete decomposition of 50.0 g of ammonium carbonate.

7.  Calculate the mass of the metal in each of the following compounds

a.  20.0 grams of chromium (II) chloride

b.  20.0 grams of chromium (III) chloride

c.  10.0 grams of copper (I) bromide

d.  10.0 grams of copper (II) bromide

8.  Calculate the mass in grams of the lead which could be obtained from the following samples of different lead oxides:

a.  50.0 grams of lead (II) oxide which is 30.o % pure.

b.  40.0 grams of lead (III) oxide which is 60.0% pure

c. 
30.0 grams of lead(IV) oxide which is 70.0% pure.

9.  In a laboratory experiment, barium chloride dihydrate is heated to remove completely its water of hydration. Calculate (a) the experimental percent of water, (b)the percent of BaCl2, and (c) the percent error. The data obtained in the experiment:

1.  empty crucible and cover 20.286 grams

2.  crucible, cover, and contents before heating 21.673 grams

3.  crucible, cover, and contents after heating 21.461 grams

10.  Calcium dihydrogen phosphate is an important fertilizer. What is the percent phosphorous in Ca(H2PO4)2.

11.  Potassium chlorate can be decomposed by heating and gives products of potassium chloride and oxygen. If a 2.45 gram sample of KClO3 shows a mass loss of 0.192 gram, what percent of original potassium chlorate has decomposed?