7. Define buffer by a. composition b. properties. What are the expected pHs of t

Question

7. Define buffer by a. composition b. properties. What are the expected pHs of the following solution of bicarbonate buffer? c. 0.45 M carbonic acid (H2CO3) with 0.45 M sodium bicarbonate (NaHCO3) d. 0. 45 M carbonic acid (H2CO3) with 0.045 M sodium bicarbonate (NaHCO3) 0.045 M carbonic acid (H2CO3) with 0. 45 M sodium bicarbonate (NaHCO3) e. f. 0.86 M carbonic acid (H2COs) with 0.86 M sodium bicarbonate (NaHCO3) g. What are the two characteristics of the best buffers? h. Analyze the four solutions above. Which is best overall buffer, and why?

Explanation / Answer

(a) A buffer is a solution that has a stable pH and is made by mixing large volumes of a weak acid and its conjugate base (actually salt of the weak acid with a strong base). This constitutes an acid buffer. A basic buffer may be similarly prepared by mixing large volumes of a weak base with its conjugate acid, which is really the salt of the weak base with a strong acid.

In a buffer solution, the weak acid and its conjugate base (or the weak base and its conjugate acid) do not neutralize each other.

(b) A buffer has a stable pH and can withstand or resist changes in pH of the solution due to addition of strong acid or base. Infact, buffers are added to solutions to keep the pH of the solution constant.

In order to answer the questions, we need to consider the dissociation of carbonic acid (H2CO3) as below.

H2CO3 (aq) ----------> H+ (aq) + HCO3- (aq); pKa1 = 6.37 …….(i)

HCO3- (aq) ----------> H+ (aq) + CO32- (aq); pKa2 = 10.32 …….(ii)

(c) The buffer solution contains 0.45 M carbonic acid (H2CO3) and 0.45 M sodium bicarbonate (NaHCO3). Use the Henderson-Hasslebach equation to obtain the pH.

pH = pKa1 + log [NaHCO3]/[H2CO3] = 6.37 + log (0.45 M)/(0.45 M) = 6.37 + log (1)

= 6.37 + (0) = 6.37 (ans).

(d) The buffer solution contains 0.45 M carbonic acid (H2CO3) and 0.045 M sodium bicarbonate (NaHCO3). Use the Henderson-Hasslebach equation to obtain the pH.

pH = pKa1 + log [NaHCO3]/[H2CO3] = 6.37 + log (0.045 M)/(0.45 M) = 6.37 + log (0.1)

= 6.37 + (-1) = 5.37 (ans).

(e) The buffer solution contains 0.045 M carbonic acid (H2CO3) and 0.45 M sodium bicarbonate (NaHCO3). Use the Henderson-Hasslebach equation to obtain the pH.

pH = pKa1 + log [NaHCO3]/[H2CO3] = 6.37 + log (0.45 M)/(0.045 M) = 6.37 + log (10)

= 6.37 + (1) = 7.37 (ans).

(f) The buffer solution contains 0.86 M carbonic acid (H2CO3) and 0.86 M sodium bicarbonate (NaHCO3). Use the Henderson-Hasslebach equation to obtain the pH.

pH = pKa1 + log [NaHCO3]/[H2CO3] = 6.37 + log (0.86 M)/(0.86 M) = 6.37 + log (1)

= 6.37 + (0) = 6.37 (ans).

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