been given an excel assignment about the dissociation of acids.

I get the whole Ka = [H+][A-]/[HA]

and from the Ka I've got the pKa value.

I'm then supposed to work out (something) against a known pH. (whatever the values are for is not listed, its just numbers)

so for example (from the "guide" spreadsheet given

Acetic acid, pKa = 4.75
pH
1.0 1584.89
1.6 398.11
2.2 100
2.8 25.12
etc

I can see how to work out pH from a given pKa and concentrations (the Henderson–Hasselbalch equation)

pH = pKa + log[A-]/[HA]

I thought rearranging it might help, but so far no luck

managed to work it out. it was the ratio between dissociated and non dissociated.

all I needed was the equation

=1/)10^(pH-pKa)

only have to work out this checksum bollocks.