Ouch, that hurts!
Bees are beautiful creatures that help plants flourish. They carry pollen from one plant to another to facilitate plant growth and development. But, they can also be troublesome when they sting you. For people who are allergic to bee venom, this can be a serious, lifethreatening problem. For the rest of us, it can be a painful experience. When stung by a bee, one firstaid treatment is to apply a paste of baking soda (sodium bicarbonate) to the stung area. This weak base helps with the itching and swelling that accompanies the bee sting.
Calculating pH of Weak Acid and Base Solutions
The
Sample Problem: Calculating the pH of a Weak Acid
Calculate the pH of a 2.00 M solution of nitrous acid (HNO
_{
2
}
). The
Step 1: List the known values and plan the problem.
Known
 initial [HNO _{ 2 } ] = 2.00 M

Ka=4.5×10−4
Unknown
 pH = ?
First, an ICE table is set up with the variable
Step 2: Solve.
Concentrations  [HNO _{ 2 } ]  [H ^{ + } ]  [NO _{ 2 } ^{ − } ] 
Initial  2.00  0  0 
Change 



Equilibrium 



The
The quadratic equation is required to solve this equation for
Since the variable
Step 3: Think about your result.
The pH of a 2.00 M solution of a strong acid would be equal to
The procedure for calculating the pH of a solution of a weak base is similar to that of the weak acid in the sample problem. However, the variable
Summary
 The procedure for calculating the pH of a weak acid or base is illustrated.
Practice
Perform the calculations at the site below:
http://www.sciencegeek.net/APchemistry/APtaters/pHcalculations.htm
Review
Questions

What does
x stand for in the equation?  What simplifying assumption is made?

What would
x stand for if we were calculating pOH?