[kapital]

Calculate ratio betwen carbonic acid and hydrogencarbonate,at pH=7,4.
(pKa1=6,4)

I tried using constant, but I get wrong Can somebody please tell me result.

[Borek]

Show what you did.

[kapital]

pKa=6.4 --> Ka = 3.9 * 10^(-7)

pH= 7.4  --> [H+]= 3.9 * 10^(-8)

[tex]Ka= \frac{[HCO_3^{-}][H^{+}]}{[H_2CO_3]}[/tex]


[tex]3.9\times10^{-7}= \frac{[HCO_3^{-}]\times 3.9 \times10^{-9}}{[H_2CO_3]}[/tex]


[tex][HCO_3^{-}]=10[H_2CO_3][/tex]

[Borek]

Everything is OK, you just have to rearrange the final result to get ratio of concentrations. Actually your second LaTeX equation contains the ratio - just move all known numbers to one side of the equation.

Don't mix tex and sub/sup tags, it doesn't work. I corrected your equations for you.

Note: when pH differs by exactly 1 from pKa, ratio of concentrations is 10 or 0.1. Always. Compare equation (2) here: http://www.titrations.info/acid-base-titration-indicators (it is about indicators, but it holds for every weak acid).

[kapital]

[tex][HCO_3^{-}]=10[H_2CO_3][/tex] -->

[tex]\frac{[H_2CO_3]}{[HCO_3^{-}]}=0.1[/tex]


But that means, it is 10 more times of carbonic acid then hydrogencarbonate, but that is wrong.

[Borek]

[tex]\frac{[H_2CO_3]}{[HCO_3^{-}]}=0.1[/tex]


But that means, it is 10 more times of carbonic acid then hydrogencarbonate, but that is wrong.

No, it means there is 10 times LESS carbonic acid.

[kapital]

Yes, I see now. Thank you.