A volume of 500.0 mL of 0.130 M NaOHNaOH is added to 625 mL of 0.200 M weak acid (Ka=8.94×10−5). What is the pH of the resulting buffer?
HA(aq)+OH−(aq)⟶H2O(l)+A−(aq)
We have , Molarity = No. of moles of solute / volume of solution in L
No. of moles of solute = Molarity
volume of solution in L
No. of moles of NaOH = 0.130 mol / L
0.500 L = 0.0650 mol
No. of moles of HA = 0.200 mol / L
0.625 L = 0.125 mol
Consider reaction, HA + NaOH
NaA + H2O
Let's use Table.
| Concentration (Moles) | HA | NaOH | NaA |
| Initial | 0.125 | 0.0650 | |
| Change | - 0.0650 | - 0.0650 | +0.0650 |
| Equilibrium | 0.0600 | 0.000 | 0.0650 |
Volume of solution = 500 + 625 = 1125 ml = 1.125 L
Solution contain weak acid HA & its salt NaA. This solution acts as a buffer solution & its pH is calculated by using Henderson's equation.
pH = pKa + log [ NaA ] / [ HA ]
pH = - log K a + log [ NaA ] / [ HA ]
pH = - log ( 8.94
10 -05 ) + log ( 0.0650 mol / 1.125 L ) / ( 0.0600 mol
/ 1.125 L )
pH = 4.05 + 0.0348
pH = 4.08
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