Hi, I really need help with this its very confusing since I cant identify the weak acid and its conjugate base...I think its H3PO4 and H2PO4- but there's no initial concentration of H3PO4 so how do I work this out:
A buffer is prepared by adding 150mL of 1.0M NaOH to 250mL of 1.0M NaH2PO4. How many moles of HCl must be added to this buffer solution to change the pH by 0.18 units?
Please help, any input would be very much appreciated.
2007-06-08 06:20:46 · 3 answers · asked by Anonymous in Science & Mathematics ➔ Chemistry
Answer is 0.25.
2007-06-08 06:29:13 · update #1
Ka values are not given, is this normal
2007-06-08 06:30:39 · update #2
In the initial buffer, the conjugate acid and base pair are H2PO4- and HPO42-. This is because you are beginning with H2PO4- ions and adding NaOH, which will convert some of this to HPO42- ions.
You will need to look up the pKa for the H2PO4- ionization in order to solve the problem.
Start with the Henderson-Haselbalch equation and calculate the pH of the original buffer solution. Then, calculate the ratio of HPO42-/H2PO4- at the lower pH. Finally, you can calculate how much HCl had to be added to get that ratio from the initial ratio.
2007-06-08 06:34:55 · answer #1 · answered by hcbiochem 7 · 1⤊ 0⤋
The NaOH converts 150/1000 moles of NaH2PO4 to Na2HPO4. The buffer is therefore (H2PO4)- and (HPO4)2-.
You have to work out how much NaH2PO4 is left, how much Na2HPO4 is formed, and you use the 2nd Ka value of H3PO4, not the first.
2007-06-08 06:27:06 · answer #2 · answered by Gervald F 7 · 0⤊ 0⤋
0.45
2007-06-08 06:26:28 · answer #3 · answered by ag_iitkgp 7 · 0⤊ 1⤋