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    Calculate the pH of the buffer solution formed when 10.00 cm3 of
    0.100 mol dm–3 potassium hydroxide are added to 25.00 cm3 of 0.410 mol dm–3 ethanoic acid

    Jan 2010.

    I got a ph of 4.80, but the answer is 3.79. I calculated it correct up to the point of the equation, and I am still confused as to why you do no need to calculate the new CONCENTRATIONS by dividing by the new total volume of 35cm^3

    Why do you find new concentrations by dividing by the total volume and when do you not?
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    Check back on your calculations. I did the same thing as you haha. In this case, it doesn't matter if you divide by the new volume because they will just cancel out. The reason you got 4.80 (And so did I at first) is because you divided by 0.01 moles instead of 0.001 moles. Usually in this case when it gives you two solutions, you add them up and divide by the total volume. Its the safest bet because you will just end up with the same answer (In this question, not all of them: Some equilibrium constant questions might not allow them to cancel out)

    Hope this helps!
 
 
 
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