pH of a solution

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reising1

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EK 1001 Chem

827. If 30 mL of a 3 M acetic acid (pKa = 4.7) were added to 45 mL of a 2 M sodium hydroxide, the resulting solution would have a pH:
A. less than two
B. between two and seven
C. equal to seven
D. greater than seven
Answer: D

When I did this problem, I thought it was equal to seven, since there is 90 mmol of acid and 90 mmol of base. Why is the answer D? Is it that we are supposed to recognize that acetic acid is a weak acid and so thus the NaOH "overpowers" the pH?
 
EK 1001 Chem

827. If 30 mL of a 3 M acetic acid (pKa = 4.7) were added to 45 mL of a 2 M sodium hydroxide, the resulting solution would have a pH:
A. less than two
B. between two and seven
C. equal to seven
D. greater than seven
Answer: D

When I did this problem, I thought it was equal to seven, since there is 90 mmol of acid and 90 mmol of base. Why is the answer D? Is it that we are supposed to recognize that acetic acid is a weak acid and so thus the NaOH "overpowers" the pH?

You have the right reasoning but I think you just picked the wrong answer.

You have 90 mmol of each acid and base.. but as you said, the weak acid won't neutralize all of the strong base. Thus, there will be extra base left in the solution resulting in a pH greater than neutral (i.e. greater than seven).
 
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