A 200.0 mL solution of 0.40 M CH3COOH(aq) and 0.40 M NaCH3COO(aq) has 20.0 mL of 1.0 M HCl(aq) added to it. What is the pH after the HCl has been added? (Ka = 1.8 x 10-5 for CH3COOH)
You start with 80 mmole each of HA and A-, which makes it a buffer with a pH = pKa (4.74 in this case). To that you've added 20 mmoles of HCl, so after reaction you have a buffer with 60 mmoles A- and 100 mmoles HA (rich in conjugate acid). It makes the pH slightly less than the pKa, but not as much lower as log 2. So we know the pH is less than 4.74 but not as low as 4.44 (found from 4.74 - 0.30). You're looking for the answer choice in that range, but closer to 4.44 than 4.74.
What are the four answer choices that come with this MCAT practice question?
The key thing to recognize here is that you're starting with a buffer of a pH between 4.5 and 5 (you wouldn't be expected on the MCAT to be able to calculate this exactly) and that you're not adding that much acid - so you're still in the buffer region. Meaning the pH is still close to where you started.
The key thing to recognize here is that you're starting with a buffer of a pH between 4.5 and 5 (you wouldn't be expected on the MCAT to be able to calculate this exactly) and that you're not adding that much acid - so you're still in the buffer region. Meaning the pH is still close to where you started.
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