Phosphoric acid (H3PO4) is a polyprotic acid with pKa1 = 2.2; pKa2 = 6.8; pKa3 =
ID: 926194 • Letter: P
Question
Phosphoric acid (H3PO4) is a polyprotic acid with pKa1 = 2.2; pKa2 = 6.8; pKa3 = 12.4. The conjugate base forms are called: monosodium phosphate, disodium phosphate, and trisodium phosphate. You want to make a buffer at pH 6.8, but you have run out of disodium phosphate. Which of the following choices could you add to a solution of 0.1M trisodium phosphate to make the desired buffer?
1.5 equivalent of HCl
1.0 equivalent of NaOH
None of the above.
2.0 equivalent of NaOH
1.5 equivalent of NaOH
2.0 equivalent of HCl
1.0 equivalent of H3PO4
2.0 equivalent of H3PO4
1.0 equivalent of HCl
1.5 equivalent of H3PO4
Explanation / Answer
We can not obtain the buffer with the desired pH without the use of H2PO4 as the desired pH range is near to pka of that acid only.