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Experiment_11_-_buffor_solutions

# Experiment_11_-_buffor_solutions - 11 Buffer Solutions...

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11 Buffer Solutions Introduction Any solution that contains both a weak acid HA and its conjugate base A in significant amounts is a buffer solution. A buffer is a solution that will tend to maintain its pH when small amounts of either acid or base are added to it. Buffer solutions can be made to maintain almost any pH, depending on the acid-base pair used. The pH of a buffer solution is determined by the K a of the acid and by the ratio of concentrations of HA and A . This can be calculated by rearranging the expression for the K a of the conjugate acid of the buffer: K a = [H 3 O + ][A - ] [HA] becomes [H 3 O + ] = K a [HA] [A - ] The rearranged equation shows that the H 3 O + ion concentration of the buffer solution can be found by multiplying the K a of the acid by the ratio of the molar concentrations of the two components. To solve directly for the pH of the buffer, the equation can be put into logarithmic form. If the above equation is rearranged and the negative log of both sides is taken, a new form of the equation known as the Henderson-Hasselbalch equation results. pH = pK a + log [A - ] [HA] Henderson-Hasselbalch equation A buffer solution can maintain its approximate pH when an acid or a base is added to it because it can react with both acids and bases. If a strong acid (H 3 O + ) is added, the basic component of the buffer (A ) can react with it, and if a strong base (OH ) is added, the acidic component of the buffer (HA) will react with it. H 3 O + ( aq ) + A ( aq ) → HA ( aq ) + H 2 O ( l ) OH (aq) + HA (aq) → H 2 O ( l ) + A ( aq ) In this way, any strong acid or strong base that is added to the buffer solution is converted into a weak acid or weak base. The ratio of weak acid to weak base changes, which causes the pH to change slightly, but not drastically. Chemistry 1B Experiment 11 49

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Buffer solutions are most effective when both components, the conjugate acid and the conjugate base, are present in reasonably large concentrations. If this is the case, the buffer is said to have a high buffer capacity. Also, a buffer is most effective when there are approximately equal concentrations of the two buffer components (a ratio of [X ]/ [HX] close to 1/1) because in this case the solution will guard against large pH changes equally well whether acid is added or base is added. In order for the solution to be considered a buffer, this ratio must be between a 1/10 and a 10/1 ratio of [X - ]/[HX]. This restriction means that the [H 3 O + ] in the buffer will be within a factor of 10 of the K a value of the conjugate acid. It also means that the pH of the buffer will be within one pH unit of the value of the pK a of the conjugate acid. For example, suppose you wanted to make a buffer containing phosphoric acid (H 3 PO 4 ) and dihydrogen phosphate (H 2 PO 4 ). Since the K a of H 3 PO 4 is 7.5 × 10 -3 and the pK a is 2.12, as shown in the table below, the pH of this buffer would have to be between 1.12 and 3.12. If you wanted to make a buffer with a pH outside of this range, you would have to choose a different conjugate acid-base pair.
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