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.
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
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