Unifying concepts question OCR

This question is in the Jan 04 paper. The answer don't seem to make sense in the mark scheme.

H2CO3 ----> H+ + HCO3-
Ka = 4.17 x 10^-7 mol dm^-3

Calculate the ratio HCO3- / H2CO3 in healthy blood with a pH of 7.40.

The answer the mark scheme gives below the Ka rearrangement makes no sense. Can anyone help me?

Reply 1

Aired

Well first of all you write out the expression for Ka, and you need to rearrange it to give an expression for [HCO₃^-]/[H₂CO₃]

To do this, you have to divide both sides by [H⁺], which gives you the expression shown in the markscheme.

You can then work out [H⁺] by taking the inverse log of -7.40, and you've been given the value of Ka. :smile: