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MCAT

Unit 3: Lesson 1

Chemical and physical sciences practice passage questions

How do organisms maintain a constant pH range?

Problem

Controlling the environmental pH is important for living systems as many organisms can only exist within in a narrow pH range. Human plasma, for example, must be maintained at a pH within half a pH unit of 7, point, 4. One such mechanism that helps humans accomplish this is the phosphate buffer system inside the cytoplasm of cells. This buffer system consists of dihydrogen phosphate ions left parenthesis, start text, H, end text, start subscript, 2, end subscript, start text, P, O, end text, start subscript, 4, end subscript, start superscript, minus, end superscript, right parenthesis as hydrogen-ion donor (acid) and hydrogen phosphate ions left parenthesis, start text, H, end text, start text, P, O, end text, start subscript, 4, end subscript, start superscript, 2, minus, end superscript, right parenthesis as hydrogen-ion acceptor (base). These two ions are in equilibrium with each other as indicated by the chemical equation in Figure 1.
Figure 1. The phosphate buffer system
start text, H, end text, start subscript, 2, end subscript, start text, P, O, end text, start subscript, 4, end subscript, start superscript, minus, end superscript, left parenthesis, a, q, right parenthesisstart text, H, end text, start superscript, plus, end superscript, left parenthesis, a, q, right parenthesis + start text, H, end text, start text, P, O, end text, start subscript, 4, end subscript, start superscript, 2, minus, end superscript, left parenthesis, a, q, right parenthesis
If additional hydrogen ions enter the cellular fluid, they are consumed in the reaction with start text, H, end text, start text, P, O, end text, start subscript, 4, end subscript, start superscript, 2, minus, end superscript, and the equilibrium shifts to the left. If additional hydroxide ions enter the cellular fluid, they react with start text, H, end text, start subscript, 2, end subscript, start text, P, O, end text, start subscript, 4, end subscript, start superscript, minus, end superscript, producing start text, H, end text, start text, P, O, end text, start subscript, 4, end subscript, start superscript, 2, minus, end superscript, and shifting the equilibrium to the right. This behavior allows the pH of an aqueous buffer solution to change only slightly after drops of acid or base are added. The equilibrium-constant expression for this equilibrium is shown in Figure 2.
Figure 2. The value of start text, K, end text, start subscript, a, end subscript for this equilibrium is 6, point, 23, ×, 10, start superscript, minus, 8, end superscript at 25, °, C, which yields a start text, p, K, end text, start subscript, a, end subscript of 7, point, 21.
start text, K, end text, start subscript, a, end subscript, equals, start fraction, open bracket, start text, H, end text, start superscript, plus, end superscript, close bracket, open bracket, start text, H, end text, start text, P, O, end text, start subscript, 4, end subscript, start superscript, 2, minus, end superscript, close bracket, divided by, open bracket, start text, H, end text, start subscript, 2, end subscript, start text, P, O, end text, start subscript, 4, end subscript, start superscript, minus, end superscript, close bracket, end fraction
This equation can be reorganized in the form of the Henderson-Hasselbalch equation:
start text, p, H, end text equals start text, p, K, end text, start subscript, a, end subscript + start text, l, o, g, end text left parenthesis, open bracket, start text, H, end text, start text, P, O, end text, start subscript, 4, end subscript, start superscript, 2, minus, end superscript, close bracket, slash, open bracket, start text, H, end text, start subscript, 2, end subscript, start text, P, O, end text, start subscript, 4, end subscript, start superscript, minus, end superscript, close bracket, right parenthesis
In order to make a buffer, a weak acid must be chosen that has a start text, p, K, end text, start subscript, a, end subscript value within one unit of the the desired pH. This weak acid is then mixed with its conjugate base in equal concentration amounts. Table 1 lists a series of weak acids and conjugate bases, along with the start text, p, K, end text, start subscript, a, end subscript values for each acid.
Table 1.
Weak AcidConjugate Basestart text, p, K, end text, start subscript, a, end subscript value
start text, H, F, end textstart text, N, a, F, end text3, point, 19
start text, H, end text, start subscript, 3, end subscript, start text, C, C, O, C, O, end text, start subscript, 2, end subscript, start text, H, end textstart text, H, end text, start subscript, 3, end subscript, start text, C, C, O, C, O, end text, start subscript, 2, end subscript, start text, N, a, end text3, point, 86
start text, H, end text, start subscript, 3, end subscript, start text, C, C, O, end text, start subscript, 2, end subscript, start text, H, end textstart text, H, end text, start subscript, 3, end subscript, start text, C, C, O, end text, start subscript, 2, end subscript, start text, N, a, end text4, point, 74
start text, H, C, l, O, end textstart text, K, C, l, O, end text7, point, 49
start text, N, H, end text, start subscript, 4, end subscript, start text, C, l, end textstart text, N, H, end text, start subscript, 3, end subscript9, point, 26
Which solution has the smallest ratio of conjugate base to conjugate acid?
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