These were two questions from an online quiz I\'ve just taken, and they were bot

Question

These were two questions from an online quiz I've just taken, and they were both marked incorrect. My understanding was that the pH=pKa+ ln(A-/HA) which simplifies down to being pH=pKa, therefore the answer to question 5 using 3 sigfigs should be 4.70 and the answer to question 6 should equal 4.7. What did I do wrong? Help!

You have just made a solution by combining 50.0 mL of a 0.100 M sodium acetate solution with 150 mL of 1.00 M acetic acid (pKa = 4.7). What is the pH of the resulting solution? (Use proper sig figs. -----> 3 sigs) What is the pH of a solution containing 0.2 M acetic acid (pKa = 4.7) and 0.1 M sodium acetate? (Use 2 significant digits and no units).

Explanation / Answer

5) [A-]= 50*.1/200=.025M

[HA]= 150*1/200=.75 M

PH= 4.7+ log(.025/.75)=4.7-1.477=3.22

6)PH= 4.7 +log(.1/.2)= 4.398

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