Question

Consider a buffer solution that contains 0.30 M H3PO4 and 0.10 M NaH2PO4. pKa(H3PO4)=2.16.

1. Calculate its pH

2. Calculate the change in pH if 0.150 g of solid NaOH is added to 200 mL of this solution.

3. If the acceptable buffer range of the solution is ±0.10 pH units, calculate how many moles of H3O+ can be neutralized by 250 mL of the initial buffer.

Answer 1

1)

concnetration of acid = 0.30 M

concentration of salt = 0.10 M

pH = pKa + log [salt / acid]

     = 2.16 + log [0.1 / 0.30]

pH = 1.68

2)

moles of NaOH = 0.150 / 40 = 0.00375

moles of acid = 0.06

moles of salt = 0.02

pH = pKa + log [salt + C / acid - C]

     = 2.16 + log [0.02 + 0.00375 / 0.06 - 0.00375]

pH = 1.79

3)

pH = 1.68 - 0.10 = 1.58

1.58 = 2.16 + log [25- x / 75 + x]

[25 - x / 75 + x] = 0.263

25 - x = 19.72 - 0.263 x

x = 7.155

moles of H3O+ = 7.16 x 10^-3 mol