Chemistry Lab Now let’s find out what is the pH if half of the NaOH is added, in

Question

Chemistry Lab

Now let’s find out what is the pH if half of the NaOH is added, in another words you have added 12.5mL of 0.01M NaOH solution to 25.0mL of 0.01M of acetic acid. There is a simple way to get the answer. You don’t need to set up the mass conservation equilibrium. Think about what is the relationship between the concentration of acetate anion H3C2O2 - (aq) and acetic acid in the mixture when half of required NaOH is added. What pH of the solution will be if exactly 12.50mL of 0.01M NaOH solution is added to 25.0mL of 0.01M of acetic acid solution?

Explanation / Answer

When we add exactly half of the base, it is able to neutralize only half of CH3COOH, forming half of conjugate base CH3COO-. Since one - half of the acid is neutralized, this point is called half - neutralization point.

Hence at half - neutralization point [CH3COOH] = [CH3COONa]

Now applying Hendersen equation,

pH = pKa + log[CH3COONa] / [CH3COOH] = pKa + log 1 = pKa

=> pH = pKa = 4.74 (answer)

Hence at half - neutralization point pH = pKa

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