Buffer solution explanation question

http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/June%202012%20-%20QP/6CH04_01_que_20120613.pdf

Question 14 B (ii)
"*(ii) Explain how this buffer solution resists a change in pH when a few drops of sodium hydroxide are added." 3 marks

It is a buffer of ethanoic acid and sodium ethanoate

http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/June%202012%20-%20MS/6CH04_01_rms_20120816.pdf


The third mark is for saying that the ratio of [CH₃COOH] to [CH₃COO^-] remains fairly constant and IGNORE concentration of Hydrogen ions remains constant, why would that be ignored?

Any help will be greatly appreciated

because it's implicit in the fact that the pH does not hardly change ...

Why does pH of the buffer doesn't change? Because its pH doesn't change.

Would you accept this answer? That's basically what saying "concentration of Hydrogen ions remains constant" means in the context of the question.

Small change in the ratio is a CAUSE, small change of pH in the EFFECT. Question mentions the EFFECT and asks for CAUSE. Listing EFFECT as CAUSE won't work.