How to make a sodium carbonate/sodium hydrogen carbonate buffer

[chem_is_lovex]

Homework Statement

"Plan how to make 100mL of a buffer solution with a pH of 10.8 to be made using only sodium carbonate, sodium hydrogen carbonate and water"

Homework Equations

pH= pKa+ log(CO₃^2-)/(HCO₃^-)

The Attempt at a Solution

I tried rearranging it, but I'm not sure what to do to find both of the concentrations...

thanks :)

 

[Borek]

Calculate ratio of concentrations of carbonate and hydrogencarbonate, see how you can obtain this ratio mixing given reagents.

 

[chem_is_lovex]

Calculate ratio of concentrations of carbonate and hydrogencarbonate, see how you can obtain this ratio mixing given reagents.

ok, I did that and I got the ratio to be 3.55. I then tried to find the conc. of H⁺, therefore the conc. of carbonate ions... is this how you're supposed to do it?

it also says "you should specify the amounts of sodium carbonate and sodium hydrogen carbonate that you would use"


thanks

 

[Borek]

No, once you know ratio of carbonate and hydrogencarbonate it is just a trivial calculation of concentrations - ignore dissociation and equilibrium, just assume you are preparing some volume of the buffer, you want it to be - say - 0.01M in carbonate, use the ratio you already know to calculate concentration of hydrogencarbonate, then calculate how much of each solid you need.

 

[chem_is_lovex]

ok :) thanks! it was a lot simpler than I imagined it to be...