[scarpia]

What mass of Na2CO3 must be added to 1.5 L of NaHCO3 solution (c=0.2 mol/L) in order to prepare buffer with pH=10? Ka(HCO3-)=4.7*10^-11 mol/L


Even don't now to write proper equilibrium :/

[Borek]

This is a direct application of the Henderson-Hasselbalch equation.

[scarpia]

Indeed, but I need the equilibrium. I suppose that NaHCO3 iz acid, therefore Na2CO3 is base? Using H-H eq. I can calculate [Na2CO3] but how can I find out its mass?

[UG]

Well if you can find the concentration, and you also know the volume of the solution (1.5 L), you can calculate number of moles of sodium carbonate. Then using the molar mass, you can calculate the mass

[Arkcon]

Indeed, but I need the equilibrium. I suppose that NaHCO3 iz acid, therefore Na2CO3 is base? Using H-H eq. I can calculate [Na2CO3] but how can I find out its mass?

Good.  You noticed the H in the formula as the source of acid.  Now, can you write the Henderson-Haselbach equation, and begin to plug in known values?  Then you'll see what may (or may not) be missing, and ask for that one directly.

[scarpia]

This is my work, but I need equilibrium, what happens when you mix NaCO3 + NaHCO3 ------>

[Arkcon]

You've done some algebra with the formula, and that's OK.  But lets stick with the first one.  The values in the brackets [salt] stands for concentration, and you have one, and you need one.  I don't know why you keep asking for an equilibrium, this formula doesn't use that.

[scarpia]

Hm, mostly because my teacher insists on them :/

[Arkcon]

In that case, since this is a buffer problem, what's needed is a different equilibrium. Can you think of some other ions that would be more appropriate?  What other equilibria would describe what a buffer does?

[scarpia]

maybe

NaHCO₃ <----> Na⁺ + HCO₃^-
HCO₃- <----> H⁺ + CO₃^2-

[Arkcon]

Yes, I was hoping you'd get the second one on your own.  That is the equilibria that describes what this buffer does.

[scarpia]

Then, problem solved, thanks