Name _____________________________ Period______ Date __________
Honors Biology: ACIDS, BASES & BUFFERS
pH scale (power of Hydrogen) Range: 1 ------------------------7---------------------------14
acid
neutral
base
ACID
Substance that increases the relative
[H+] of a solution
[H+] > [OH-]
Also removes OH- because it tends to
combine with H+ to form H2O
Ex: HCl  H+ + Cl- (strong acid)
BASE
Substance that reduces the relative
[H+] of a solution
[H+]< [OH-]
May alternatively increase [OH-]
Ex. NaOH  Na+ + OH- (strong base)
OH- + H+ H2O
At equilibrium (and 25oC)—means a pH of 7
Number of H+ ions = number of OH- ions
[H+] = [OH-] = ____1____
M = 1 x 10-7 M
10,000,000
Note the brackets indicate molar concentration
pH = - log [H+] expressed in moles per liter (M) or negative log 10 of the [H+]
so at pH of 7 or neutral [H+] = 1 x 10-7 therefore
-log 10-7 = -(-7) = 7
[H+] [OH-] = 1.0 x 10-14 or often written as 10-14
In a neutral solution [H+] = 10-7M and [OH-] 10-7 M
In an acidic solution (pH = 5) where the [H+] = 10-5 M and [OH-] = 10-9 M
In a basic solution (pH = 9) where [H+] = 10-9 M and [OH-] = 10-5 M
Most biological fluids are within the pH range of 6 to 8. Few examples – stomach acid at pH = 1.5
Each pH represents a TEN fold difference (scale is logarithmic), so a slight change in pH represents a large
change in actual [H+].
EX.: How much greater is the [H+] in a solution with pH 2 than a solution with pH 6?
Answer:
pH 2 = [H+] of 1.0 x 10-2 which is 0.01M or 1/100 M
pH 6 = [H+] of 1.0 x 10-6 which is 0.000001 or 1/1,000,000M
so ANSWER is: 10,000 X greater
Problems for you:
1. What does it mean (in regard to pH) when the H+ concentration in the blood begins to fall?
2.
How many more hydrogen ions are in a solution with pH of 3 compared to pH of 5?
3. HF (hydrogen fluoride) is dissolved in pure water. Predict whether the pH of the solution will be
GREATER than or LESS than 7.0 (circle one)
4. Measurements show that the pH of a particular lake is 4.0. What is the hydrogen ion concentration of the lake?
A. 4.0 M
B. 10-10 M
C. 10-4 M
D. 104 M
E. 4%
5. Write out this number as a decimal. ___________________________________________
6. What is the hydroxide ion concentration of the lake described in question 3?
A. 10-10
B. 10-4M
C. 10-7 M
D.10-14
E. 10M
7. A solution contains 0.000001 (10-6) moles of hydroxyl ions [OH-] per liter. Which of the following best describes
this solution?
A. acidic; H+ acceptor
D. basic; H+ donor
B. basic; H+ acceptor
E. neutral
C. acidic; H+ donor
8. Silica is a hard, glossy material that does not dissolve in water. Suppose silica crystals are accidentally mixed
with sodium chloride. A. What is sodium chloride? B. Describe how you can separate the mixture.
BUFFERS = substances that minimize large sudden changes in pH. They help organisms maintain the pH of body
fluids within the narrow range necessary for life.
Are combinations of H+ donor and H+ acceptor forms in a solution on weak acids or bases.
Work by accepting H+ ions from solution when they are in excess and donating H+ ions to the solution
when they have been depleted.
Example:
Response to a rise in pH
H2CO3
H+ donor
(weak acid)
----------------------------------- HCO3
--------------------------------- H+ acceptor
Response to a drop in pH
(weak base)
+ H+
hydrogen
ion
Problems:
1. Using the equation above, if the pH of the blood drops, one would expect:
A. A decrease in the concentration of H2CO3 and an increase in the concentration of HCO3B. The concentration of hydroxide ion (OH-) to increase
C. The concentration of bicarbonate ion (HCO3-) t o increase
D. The HCO3- to act as a base and remove excess H+ with the formation of H2CO3
E. The HCO3- to act as a acid and remove excess H+ with the formation of H2CO3
2.
Carbon dioxide (CO2) is readily soluble in water according to the equation CO2 + H20 < H2CO3
Carbonic acid (H2CO3) is a weak acid. Respiring cells release CO2 into the bloodstream. What will the
effect on pH of blood as that blood first comes in contact with respiring cells?
A. Blood pH will decrease slightly.
B. Blood pH will increase slightly.
C. Blood pH will remain unchanged.
D. Blood pH will first increase, and then decrease as CO2 combines with hemoglobin.
E. Blood pH will first decrease, and then increase sharply as CO2 combines with hemoglobin.