pH question

[Ryuu-jin-jakka]

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Source: Dot Point chemistry, pg68, 3.1.3 c)
A 0.1 molL-1 solution of carbonic acid if only 8% ionises, answer 2.1

have no idae how they got the answer the only way i got to the answer if i took the chemical equation as HCO3 which i no is wrong.... ><'

[Drongoski]

Originally Posted by Ryuu-jin-jakka

Source: Dot Point chemistry, pg68, 3.1.3 c)
A 0.1 molL-1 solution of carbonic acid if only 8% ionises, answer 2.1

have no idae how they got the answer the only way i got to the answer if i took the chemical equation as HCO3 which i no is wrong.... ><'

Isn't carbonic acid: H₂CO₃ ?

[Ryuu-jin-jakka]

that what i thought too, this is my working out
= 0.1x2 (because its a diprotic) x 8/100
= 0.016
= -log[0.016]
pH = 1.8 (1.d.p)
but the answer is 2.1 so don't no what i'm doing wrong ><'

[shady145]

answers arnt always correct, dnt count courself out if the answers are different.
try to get what they got in the answers. they had carbonic acid with only 1 hydrogen... .1x.08=.008
-log.008=2.1
ur answer is correct

[brenton1987]

Originally Posted by ryuu-jin-jakka

source: Dot point chemistry, pg68, 3.1.3 c)
a 0.1 moll-1 solution of carbonic acid if only 8% ionises, answer 2.1

have no idae how they got the answer the only way i got to the answer if i took the chemical equation as hco3 which i no is wrong.... ><'

that what i thought too, this is my working out
= 0.1x2 (because its a diprotic) x 8/100
= 0.016
= -log[0.016]
ph = 1.8 (1.d.p)
but the answer is 2.1 so don't no what i'm doing wrong ><'

The dissociation of carbonic acid occurs in two steps.
H₂CO₃ --> H⁺ + HCO₃^-
HCO₃^- --> H⁺ + CO₃^2-

For multiprotic acids the first proton is removed on all of the molecules before the second proton is removed. So in the formula above, reaction 1 will occur completely before reaction two begins. This is why when titrating a diprotic acid you get two equivalence points. The first point is caused by reaction one, the second by reaction two.

Now, the question says '8% ionised'. This means that reaction 1 occurs to 8% completion but reaction 2 remains at 0%.

Thus,
pH = -log₁₀[H+]
pH = -log₁₀(0.1 * 8 / 100)
pH = 2.1

[mR sinister]

^
I understand what your saying.... but i dont think it occurs in ALL multiprotic acids.
If they are strong acids eg H2SO4
They have a 100% degree of ionisation, so it would only take 1 reaction to completely transfer all of its protons.

So i guess the man problem for you in this question was realizing carbonic acid is a WEAK Diprotic acid.

[brenton1987]

Originally Posted by mR sinister

If they are strong acids eg H2SO4
They have a 100% degree of ionisation, so it would only take 1 reaction to completely transfer all of its protons.

Sulfuric acid is only 100% dissociated for the first proton. The second proton is much weaker.