A beaker with 175 mL of an acetic acid buffer with a pH of 5.000 is sitting on a benchtop. The total molarity of acid
ID: 956457 • Letter: A
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and conjugate base in this buffer is 0.100 M. A student adds 6.80 mL of a 0.490 M HCl solution to the beaker. How much will the pH change? The pKa of acetic acid is 4.740. Express your answer numerically to two decimal places. Use a minus (-) sign if the pH has decreased. A beaker with 175 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 6.80 mL of a 0.490 M HCl solution to the beaker. How much will the pH change? The pKa of acetic acid is 4.740. Express your answer numerically to two decimal places. Use a minus (-) sign if the pH has decreased. A beaker with 175 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 6.80 mL of a 0.490 M HCl solution to the beaker. How much will the pH change? The pKa of acetic acid is 4.740. Express your answer numerically to two decimal places. Use a minus (-) sign if the pH has decreased. and conjugate base in this buffer is 0.100 M. A student adds 6.80 mL of a 0.490 M HCl solution to the beaker. How much will the pH change? The pKa of acetic acid is 4.740. Express your answer numerically to two decimal places. Use a minus (-) sign if the pH has decreased.Explanation / Answer
mmol of HCl added = MV = 6.8*0.49 = 3.332 mmol of HCl
then we need mmol of acetic acid and acetate
from pH equation
pH = pKa + log(acetate/conjugate)
since pKa = 4.75 and pH = 5
then
5 = 4.75 + log(acetate/acetic acid)
ratio = 10^*(5-4.75) = 1.778
acetate = 1.778*acid
if acetate = 0.1 then
acid = 1.778
then
mmol of aceteic acid = 1.778*175 = 311.15
mmol of acetate = 0.1*175 = 17.5
then, after adding 3.332 mmol of HCl
mmol of aceteic acid = 311.15+3.332 = 314.482
mmol of acetate = 17.5-3.332 = 14.168
then
pH = 4.75 + log(14.168/314.482)
pH = 3.403
the pH decrase = 5-3.4 = 1.6 pH units decreae i.e -1.6
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