Can somebody explain to me a 2008 question? :(

[chickenpop]

I know we get the Ka from the data book, but what does it actually do?
And why are we multiplying the [H+] by the percentage ionisation?

An explanation of each of the steps would be greatly appreciated, cause I have no idea what's going on for this question

 

[scocliffe09]

line 1 is just, as you said, Ka value taken from data book.
line 2 is [H+] = 10^-pH. the value given in brackets is just 10^-2.1 once you chuck it in the calculator
line 3: next we assume that because ionisation is a one:one reaction, [H+]=[HCOO-]. then you just sub the value calculated in line 2 for [H+] and the value for Ka from line 1 into the Ka expression (top line of Ka expression becomes [H+]^2)
the rest is just rearranging and calculater work. hope that helps

[chickenpop]

ah, I see now. Thanks scocliffe09