chemistry help?? Calculating pH of a weak acid?

Question

Example 1:
Calculate the percentage of HF molecules ionized in a 0.10 M HF solution. (Ka = 6.8 x 10-4)

*I used the ICE chart, but was not able to know how to apply the 5% rule to this equation. What is the formula for the 5%rule?*

 

 

Answer 1

Step 1:

The concentration of the [HF] is 0.100 M.

k_a for HF is .

The chemical equation is .

Let us consider change in the concentration:  , then .

Step 2:

Acid ionization factor .

Concentration cannot be negative.

Step 3:

Percentage ionization :

.

Solution:

.