pH and pOH

The pH Scale pH is just another way to express the hydrogen ion concentration ([H+]), of an acidic or basic solution.

What about pH? pH is defined as the negative log of hydrogen ion concentration. Mathematically it looks like this: pH = - log [H+]

Try these on your calculator [H+] pH 1 ×10-3 2.5 ×10-11 4.7 ×10-9 5.8 ×10-4 1.0×10-7

pH values Acids Bases Neutral solutions pH < 7 pH > 7 pH = 7 The lower the pH, the stronger the acid The higher the pH, the stronger the base

Examples of Calculating pH 1.Calculate the pH of a 0.01M HNO3 solution? 

Example #2 Find the pH of a 0.01 M solution of ammonia. Ammonia is a weak base with Kb= 1.8 × 10-5

pOH  There is a way to simplify the last parts of this operation, by using pOH: pOH = - log [OH-] Once we find [OH-] for a base, we can determine pOH: [OH-] =4.2 × 10-4 pOH= -log [OH-]  = -log (4.2×10-4)  pOH= 3.4 answer Next we use of the following easy-to-memorize relationship: pH + pOH = 14 Once we find pOH, it is a simple matter to find pH: OR pH = 14 - pOH   = 14 - 3.4  pH= 10.6

It doesn't matter which method you use to find [H+] and pH for a base - both will give you the same answer. Choose whichever method works best for you.

Do questions 8,9,11 from the practice problems 4-2-4

Finding [H+] when you know pH You know the pH of a solution and need to find [H+], or the concentration of the acid solution. How do you do that? To convert pH into [H+] involves taking the antilog of the negative value of pH .[H+] = antilog (-pH) different calculators work slightly differently - make sure you can do the following calculations using your calculator. Practice as we go along . . .

Example. We have a solution with a pH = 8.3. What is [H+]? Enter 8.3 as a negative number Use your calculator's 2nd or Shift or INV key to type in the symbol found above the LOG key. The shifted function should be 10x. You should get the answer 5.0 × 10-9

Other calculators require you to enter keys in the order they appear in the equation. Use the Shift or second function to key in the 10x function. Use the +/- key to type in a negative number, then type in 8.3 You should get the answer 5.0 × 10-9 If neither of these methods work, try rearranging the order in which you type in the keys.

Example 1.Find the hydronium ion concentration in a solution with a pH of 12.6. Is this solution an acid or a base? How do you know? 

Example #2 A 0.24M solution of the weak acid, H2CO3, has a pH of 3.49. Determine Ka for H2CO3 (carbonic acid). 

Assignment 5.2.4