I don't understand why for some questions I have to multiply the concentration by the total volume of both the acid and the alkali whereas in others I have to multiply by just the volume of the acid or alkali individually.
For example, in the question telling me to calculate the pH of a buffer solution by mixing weak acids with their salts.
50cm^3 of 1.00 mol/dm^3 methanoic acid (Ka = 1.78 x 10^-4) mixed with 20.0 cm^3 of 1.00 mol dm^-3 sodium methanoate
In this question, the mark scheme says to multiply 1.00 by 50/70 and 1.00 by 20/70 as I deduced how the 70 value is the combination of the two volumes. However usually I would multiply 1 by 20 and 50 respectively and then subtracting these values to obtain 30 and converting this to dm^3 I would get 0.030. This method has worked several times for exam questions us I don't understand how my method isn't viable here.