5 Steps to a 5 AP Chemistry

The pOH of a solution can also be calculated. It is defined as pOH =−log[OH−]. The

pH and the pOH are related:

In any of the problems above in which [H+] or [OH−] was calculated, you can now cal-

culate the pH or pOH of the solution.

You can estimate the pH of a solution by looking at its [H+]. For example, if a solution

has an [H+] = 1 × 10 −^5 , its pH would be 5. This value was determined from the value of

the exponent in the [H+].

Kb—The Base Dissociation Constant

Weak bases (B), when placed into water, also establish an equilibrium system much like weak acids:

B(aq) + H O(l) 2 HB (aq) + OH aq)+−(

pH pOH p+==Kw 14 00. at 25 °C

220  Step 4. Review the Knowledge You Need to Score High

pH

14

13

12

11

10

9 8 7 6 5 4 3 2 1 0

1 M NaOH

(14.0)

Household

ammonia (11.9)

Milk of magnesia

(10.5)

Detergent

solution (~10)

Seawater (7.0–8.3)

Blood (7.4)

NEUTRAL

Milk (6.4)

Urine (4.8–7.5)

Rain water (5.6)

Vinegar

(2.4–3.4)

Lemon juice

(2.2–2.4)

Stomach acid

(1.0–3.0)

1 M HCl (0.0)

MORE BASIC

MORE ACIDIC

Figure 15.2 The pH scale.