Cliffs AP Chemistry, 3rd Edition

Step 4: Determine 7ACHO 232 -

The molar ratio of [H+] to 7ACHO 23 - 2 is 1:1, 7ACHO 232 - = 0.0100 M also.

Step 5: Substitute the concentrations into the equilibrium expression.

.

.

K HC H O.

HCHO

099

0 0100

a 100 10

2

4

232

(^232)
==. #

+

^h

6

77

@

AA

5. What is the % dissociation of the acetic acid? (Use the 5% rule.)

A. 0.05%

B. 1.00%

C. 1.50%

D. 2.00%

E. 2.50%

Answer: B

Step 1: Write the generic formula for % dissociation.

%%%dissociation whole

part

M

100 M 100

HC H O available

HC H O dissociated

232

==##^232

Step 2: Substitute the known information into the generic equation and solve.

%dissociation=0 0100. 099. # 100 %.%.1 00

Note: The 5% rule states that the approximation a−x≈ais valid if x< 0.05a. The rule

depends on the generalization that the value of the constant in the equation in which x

appears is seldom known to be better than 5%.

6. Given that the first, second, and third dissociation constants for H 3 PO 4 are 7.0 × 10 –3,
   6.0 ×l0–8, and 5.0 × 10 –13, respectively, calculate Kfor the overall reaction.

A. 2.10 × 10 –32

B. 2.10 × 10 –28

C. 2.10 × 10 –22

D. 2.10 × 10 –11

E. 2.10 × 1022

Answer: C

Equilibrium