One beaker contains 10.0mL of acetic acid/sodium acetate buffer at maximum buffe

Question

One beaker contains 10.0mL of acetic acid/sodium acetate buffer at maximum buffer capacity (equal concentrations of acetic acid and sodium acetate), and another contains 10.0mL of pure water. Calculate the hydronium ion concentration and the pH after the addition of 0.25mL of 0.10M to HCl to each one. What accounts for the difference in the hydronium ion concentrations? Explain this based on equilibrium concepts; in other words, saying that “one solution is a buffer” is not sufficient.

Thank you so much!!!

Explanation / Answer

Solution :-

10 ml buffer solution with 0.1 M concentration of the acetic acid and acetate

Lets calculate the moles of each

Moles = molarity * volume in liter

Moles = 0.1 mol per L * 0.010 L = 0.001 mol acetic acid and sodium acetate

Now lets calculate the moles of HCl

Moles of HCl = 0.1 mol per L * 0.00025 L = 0.000025 mol HCl

So after adding the HCl to the buffer solution the HCl reacts with the aceate ion to form the acetic acid

So the new moles of the acetate = 0.001 mol – 0.000025 mol = 0.000975 mol

New moles of acetic acid = 0.001 mol + 0.000025 mol = 0.001025 mol

Now lets calculate the pH of the solution using the Henderson equation.

pH= pka + log [acetate/ acetic acid]

pH= 4.74 + log [0.000975 / 0.001025]

pH= 4.72

initial pH of the buffer before adding any HCl is 4.74 because the molarities of the acid and conjugate base are same therefore pH= pka = 4.74

so the pH of the solution changes from 4.74 to 4.72 after the addition of the HCl which is very little change in the pH

Therefore the buffer capacity of the buffer accounts for the hydronium ion concentration of the solution.

2)now lets calculate the pH of the solution when 0.25 ml of 0.1 M HCl is added to 10 ml pure water

new molarity of the HCl = 0.1 M * 0.25 ml / 10.25 ml = 0.002439 M

HCl is strong acid therefore it gives 100 % dissociation so the concentration of the H+ = 0.002439 M

pH= -log [H+]

pH= -log [0.002439]

pH= 2.62

so the when HCl is added to pure water then the pH is lower but when the HCl is added to the buffer solution then pH changes very little.

Related Questions

Navigate

• Browse (All)
• Browse O
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.