Practice Problems for pH
Agronomy 354

1. If the H+ concentration is 0.0001 M/l, what is the pH?

This can be rewritten as 1.0 x 10-4 M/l
since pH = - log [H+]

pH = - (-4) = 4

or place 0.0001 in your calculator and take the log = -4; since pH is - log, = -(-4) = 4

2.  If the H+ concentration is 0.00001 M/l, what is the OH- concentration?
This can be rewritten as 1.0 x 10-5 M/l
since pH = - log [H+]; and pH + pOH = 14
pH = -(-5) = 5

pOH = 14 - 5 = 9

3. If the H+ concentration is 0.00005 M/l, what is the pH?
This can be rewritten as 5.0 x 10-5
since pH = -log [H+]
pH = [-log 1 x 105.0] - [log 5.0] = [- (-5)] - [0.699)]; the log of 1 X 10-5 = -5 and the log 5.0 = 0.699.
pH = 5 - 0.699

pH = 4.3

or place 0.00005 in your calculator and take the log = -4.3; since pH is - log, = -(-4.3) = 4.3

4. If the pH is 4.5, what is the pOH?

pH + pOH = 14
pOH = 14 - 4.5 = 9.5

pOH = 9.5

5. If a soil has a pH of 4.7, what is the H+ concentration of the soil solution?

pH = -log [H+]
4.7 = -log [H+]
4.7 = -(- 5) - 0.3; 100.3=2.0 (the -5 tells you the number of positions to move the decimal point to the left of 100.3=2.0)

[H+] = 0.00002 M/l

or place -4.7 in your calculator and take the antilog (often inverse log or 10x) = 0.00002 M/l

6.  If a CEC is 20 meq/100 g soil and analyses find 12 meq Ca/100 g, 2 meq K/100 g, 1 meq Mg/100 g, 1 meq Na/100 g, 0.5 meq H/100 g, and 3.5 meq Al/100 g, what is the percentage base saturation?

%BS = meq basic ions/CEC

Basic ions = Ca, Mg, K, NA; Acidic ions = H and Al

meq BS = 12 + 2 + 1 + 1 = 16 meq/100 g soil

CEC = 12 + 2 + 1 + 1 + 0.5 + 3.5 = 20 meq /100 g soil

%BS =  16/20 = 80%

7. Recall that a 90% base saturation in a Midwest soil approximately equals a pH of 6.5. Therefore, this soil's pH would be lower; perhaps 5.8 or so. You may verify this by checking the graph given in lecture that relates pH and % base saturation for various soil components.