playing with diprotic acids

[kingjewel1]

hi there,

i was hopeful someone could clear this up for me.

if i have a solution of H2SO4(aq) and i want to calculate its pH would i count the deprotonations of both H+ ions as equal?

eg. if the concentration of my solution is 0.3mol dm-3

what is my pH?

is it?

a) -lg2[0.3]=0.22

or

b) -lg[0.3]=0.52

 

the thing is the 2 proton is obscured by the first so to say. in a way 1:10 concentration. so... would it be... c) -lg[0.33]=0.48?

 

i like the last one but i've seen both the first two before

tell us what you think

thanks

[budullewraagh]

well, they both have dissociation constants and the HSO4- is much less apt to dissociate as the first hydronium cation. look up these constants and take it from there

[jdurg]

The first proton will deprotonate nearly 100%, so when calculating the pH of H2SO4 it is common practice to just use that solitary proton. The amount of dissociation of HSO4- into H+ and SO4(2-) is negligible on the overall pH. However, if you really wanted to get anal-retentive and get the exact pH, you'd have to get the Ka of HSO4- and use the I.C.E. chart while taking into account the concentration of the H+ ions in solution due to the first deprotonation.

[kingjewel1]

LOL yup my examiners appear to have certain retention so..... the problem is that i've seen all three answers from 3 different sources. i mean you might aswell use the right answer if that's what they're looking for...

 

 

would c) be good enough though?