Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: University_Of_Toronto on October 31, 2007, 12:48:40 PM
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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?
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Na3PO4 + HCl = Na2HPO4 + NaCl
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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?