A beaker with 105 mL of an acetic acid buffer with a pH of 5.000 is sitting on a benchtop.
The total molarity of acid and conjugate base in this buffer is 0.100 M
. A student adds 5.50
mL of a 0.470 M
HCl solution to the beaker. How much will the pH change? The pK
acetic acid is 4.740.
ONE: First calculate the number of moles of conjugate acid and conjugate base in the initial buffer. Next,
determine the effect of the addition of the acid, HCl, on the total number of moles of conjugate acid and
conjugate base. Finally, use the Henderson-Hasselbalch equation to determine the new pH,
a+log[conjugate base]/[conjugate acid]
and compare this value to the initial pH of 5.00 to determine the pH change.
ONE STEP ONE: Initial mols of weak acid
Find mols of acid + base : 105 mL = 0.105 L , 0.100M , > 0.0105 mols acid and base in
Find the value of nA- / nHA = 10^ pH - pka = 1.82
Find value of nHA : nA- / nHA
acid + base = 0.0105
base/acid = 1.82
Because nA- / nHA , nA- = 1.82nHA
0.0105 = 1.82(nHA) + nA which is like 2x + x = 4
So do 0.0105 / 2.82 = 0.0037 is
ONE STEP TWO: initial mols of base
Find mols of base from previsou information.
acid + base = 0.0105 mols
acid mols = 0.0037 mols
Base mols would be = 0.006776
ONE STEP THREE: mols of HCL added to acetic acid buffer
5.50 mL HCL is 0.0055 L * 0.470 M = 0.002585 mols
ONE STEP FOUR: New mols of conjugate base
Initial base - mols of HCl = 0.006776 - 0.002585 = 0.004191 mol!!
ONE STEP FIVE: New mols of acid
Initial acid + mols of HC = 0.0037 + 0.002585 = 0.006285 mol!!!
ONE STEP SIX: New pH
pH = pKa + log conjugate base / conjugate acid
pH = 4.740 + log (0.004191 / 0.006285) = 4.56401
ONE STEP SEVEN: Change in pH due to HCl addition
New pH - initial = 4.56401 - 5 = -0.4398