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Unformatted text preview: 1.5 Question 4. Consider a solution of a weak acid at a pH equal to its pKa. By how much would the pH change, and in which direction, if we added to this solution enough base to neutralize 10% of the total acid? (Answer: .18 higher (C)) Solution: If the pH is equal to the pKa for the weak acid, the concentration of the basic and the acidic forms of the acid must be equal. Take any amount of the total acid, say 1 eq., in which case the basic form = the acidic form = 0.5 eq. If we add 0.1 eq of base, we will convert the basic form to (0.5 + 0.1) = 0.6 eq; similarly, we must reduce the acidic form to (0.5 0.1) = 0.4 eq. The new pH will have changed Answer Student response (%) Comments 0.1, higher 79 0.1, lower 3.6 0.18, higher 14 This is correct. 0.18, lower 1.1 040, higher 1.8 by log(0.6/0.4), which = 0.18 pH unit. And, since we are adding base, the pH must increase....
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This note was uploaded on 03/09/2011 for the course BISC 13141 taught by Professor Williammcclure during the Spring '10 term at USC.
- Spring '10