Answer:
The new pH after adding HCl is 7.07
Explanation:
The formula for calculating pH of a buffer is
pH = pKa + log([Conjugate base]/[Acid])
Before adding HCl,
7.55 = 7.55 + log([Conjugate base]/[Acid])
⇔ log([Conjugate base]/[Acid]) = 0
⇔ [Conjugate base] = [Acid] = 1/2 x 0.100 = 0.05 M
⇒ Mole of Conjugate base = Mole of Acid = 0.05 M x 0.12 mL = 0.006 mol
After adding HCl (3.00 mL, 1.00 M)
⇒ Mole of HCl = 0.003 x 1 = 0.003 mol)
New volume solution is 120 m L+ 3 mL = 123 mL
HCl is a strong acid, it will convert the conjugate base to acid form, or we can express
Mole of new Conjugate base = 0.006 - 0.003 = 0.003 mol
⇒ Concentration = 0.003/0.123 M
Mole of new Acid form = 0.006 + 0.003 = 0.009 mol
⇒ Concentration = 0.009/0.123 M
Use the formula
pH = pKa + log([Conjugate base]/[Acid])
= 7.55 + log(0.003 / 0.009) = 7.07
