Cliffs AP Chemistry, 3rd Edition

2. A sample of a pure compound was found to contain 1.201 grams of carbon, 0.202 grams
   of hydrogen, and 7.090 grams of chlorine. What is the empirical formula for the
   compound?

A. CHCl 3

B. CH 2 Cl

C. CH 2 Cl 2

D. CH 3 Cl

E. C 2 H 2 Cl 4

Answer: C

First, change the grams of each element to moles. You end up with 0.100 mole of carbon, 0.200
mole of hydrogen, and 0.200 mole of chlorine. This represents a 1 carbon : 2 hydrogen : 2
chlorine molar ratio.

3. Balance the following equation using the lowest possible whole-number coefficients:

NH 322 +++CuO"Cu N H O

The sum of the coefficients is

A. 9

B. 10

C. 11

D. 12

E. 13

Answer: D

Step 1: Begin balancing equations by trying suitable coefficients that will give the same num-
ber of atoms of each element on both sides of the equation. Remember to change coefficients,
not subscripts.

2 NH 32 " 1 N

Step 2: Look for elements that appear only once on each side of the equation and with equal
numbers of atoms on each side. The formulas containing these elements must have the same
coefficients.

CuO" Cu

Gravimetrics