[University_Of_Toronto]

I did a search but almost all of the postings were where you have to find pH given acid and base concentration


Here is my scenerio:

Given that your pH of buffer should be 12.2 , where the conjugate acid concentration in the final 100 mL buffer solution should be 0.05 mol/L

You are given KHP, Na₃PO₄. 12H₂O ,0.5 mol/L HCl, 0.5 mol/L NaOH. If only one pair is available as a solid i.e conjugate acid or base , the buffer must be prepared using either 0.5 mol/L HCl or NaOH to get the correct pH(12.2)

I was ask to find:

Concentration of Conjugate base for buffer

Mass of conjugate base and conjugate acid required to prepare the buffer

Volume of NaOH/HCl I need to add to the solid to get the required pH

My steps so Far:

Only one pair is present so since my pH is 12.2 I have to choose a base that closely matches my pH

I choose Na₃PO₄ 12H₂O

So I will be adding 0.5 mol/L HCl to it.

pH=pka + log( moles conjuagte base/moles acid)


Con of conjugate acid in 100 mL buffer =0.05 mol/L

therefore 0.05 mol/l * 0.1 L =0.005 mol acid

12.2 =12.15 + log(x/0.005)


moles conjugate base =0.00561 mol

What Do I do Next?

[AWK]

Na3PO4 + HCl = Na2HPO4 + NaCl

[University_Of_Toronto]

Thanks for reminding me about the rxn taking place

Ok so my goal was to find the concentration of conjugate base

from the H-H eq I found A- to be 0.00561 moles and I know final buffer sol is 100 ml

so [conjugate base] =0.00561 mol/0.1 L
                           = 0.0561 mol/L

Next step was to find mass of conjugate base and acid required to prepare buffer

Mass of conjugate base = 0.05 mol/L conjugate acid * 0.1 L = 0.005 mol

( I said 1:1 ratio)...Is this correct assumption?

0.005 mol Na₃PO₄ * 380.12 g/1 mol = 4.0331 g Na₃
PO₄
My next step is to find Mass of Conjugate acid and volume of the 0.5 mol/L HCl I have to add to get the buffer pH to 12.2

How do I proceed?