Chemistry question.... some help please???

Question

The pH of a bicarbonate-carbonic acid buffer is 7.70. Calculate the ratio of the concentration of carbonic acid (H2CO3) to that of the bicarbonate ion (HCO3-).

Ka H2CO3 = 4.2*10^-7

Kb HCO3- = 2.4*10^-8

10 points for best answer!!!

Answer 1

Since we're working with a buffer we can use the Henderson–Hasselbalch equation:

pH = pKa + log (base/acid)

In the log term base and acid can be either the number of moles of each or the concentration of each.  Since we don't know how much bicarbonate or carbonic acid was used and you want an answer in ratio of concentration we'll stick with that.

Before we begin plugging variable into our equation notice that the equation asks for pKa and we are given Ka this is a simple conversion:

pKa = -log( Ka)

Here that would be: pKa = -log(4.2x10^-7) = 6.38

Now we can start plugging in:

7.7 = 6.38 + log (base/acid)



A little rearranging and we get:

log (base/acid) = 1.32

To get rid of the log we take the antilog of both sides of the equation (that's the key that says either: 10^x, ALog, or Log^-1) and we get:

(base/acid) = 20.89

I know that seems high but the procedure is correct!

Hope this helps!

Answer 2

uh no