Question

CH₃COOH yields CH₃COO^- + H⁺

If NaCH₃COO is added to the solution then…

Will the concentration of H⁺ will increase/decrease/same? Why?

Will the concentration of CH₃COOH will increase/decrease/same? Why?

Will the concentration of OH- will increase/decrease/same? Why?

Will pKa will increase/decrease/same? Why?

Answer 1

Solution:

CH₃COOH yields CH₃COO^- and H⁺ ions.

If NaCH₃COO is added to the solution, it yields Na⁺ and CH₃COO^- ions.

1. Since, after dissociation of NaCH₃COO in the solution H⁺ ion does not form. The number of moles of H⁺ will remain same but the number of moles of CH₃COO^- ion will increase, i.e., the volume of solution will increase. So, the concentration of H⁺ will derease. ( concentration = no. of moles / volume of solution)

2. Since, after dissociation of NaCH₃COO in the solution CH₃COO^- ion forms. So, after adding NaCH₃COO the concentration of CH₃COO^- ion will increase but the number of moles of H⁺ ion is remained same. To form 1 mole of CH₃COOH 1 mole H⁺ and 1 mole CH₃COO^- ion require. Since, the number of moles of H⁺ is same, so the number of moles of CH₃COOH will remain same and the volume of solution will increase. So, the concentration of CH₃COOH will derease.

3. In the solution there is no OH^- ion.

4. Since, after adding NaCH₃COO the concentration of H⁺ ion will decrease and the concentration of CH₃COO^- ion increases. So, the pH of the solution will increase.

we know that, pKa = pH - log([CH₃COO^-] / [CH₃COOH])

The concentration of CH₃COO^- ion increases and the concentration of H⁺ ion decreases.

So, log([CH₃COO^-] / [CH₃COOH]) will increase and pH will increase.

If the amount of CH₃COOH and NaCH₃COO is same, then the increase in concentration of CH₃COO^- ion and decrease in concentration of H⁺ ion will be same. So, the pKa will remain same.