Question

1. Phosphoric acid (H3PO4) is a polyprotic acid with pKa1 = 2.2; pKa2 = 6.8; pKa3 = 12.4. What is the pH (to the nearest 0.1 pH unit) of a buffer made from mixing 90 mL of 0.1 M phosphoric acid and 10 mL of 0.1 M NaH2PO4?

2. The pKa of acetic acid is 4.76. What volume of 2 M HCl can be added to 100 mL of 0.1M acetate buffer at pH 4.76 before it is 90% titrated (i.e., is at the end of its buffering capacity)?

3. For a monoprotic weak acid/conjugate base pair, what is the mole fraction of weak acid at pH = pKa + 2?

Answer 1

A buffer is a solution containing weak acid and its salt with strong base or vice versa. Here we have Phospohric acid H3PO4 as the weak acid and NaH2PO4 as its salt with strong base NaOH. Buffer solutions tend to resist change in pH.

pH of a buffer solution is given by henderson hasselbalch equation which is -

pH = pKa + log(salt/acid) where salt/acid is the ratio of moles of salt and acid , pKa is the value -logKa where Ka is the dissciation constant of the acid corresponding to the salt formed. Here we have Ka as Ka1

millimoles of salt = vol. in mL*molarity = 10mL*0.1M = 1 millimole of salt NaH2PO4

millimoles of acid = vol. in mL*molarity = 90mL*0.1M = 9 millimoles of acid H3PO4

applying henderson hasselbalch equation we have -

pH = pKa1 + log(salt/acid) = 2.2 + log(1/9) = 1.2457 = 1.2 after rounding off