Question #156ba

1 Answer
Apr 25, 2017

Answer:

pH = 5.13

Explanation:

We will use the #"Henderson-Hasselbach"# equation to find the pH of this buffer solution

#color(white)(aaaaaaaa)##color(magenta)(pH = pKa + log[("conjugate base")/("weak acid"]]#

#"Step 1"#
Find the equilibrium concentrations of the #"acetic acid"#(weak acid) and its conjugate base, #"sodium acetate"#. Find moles of each solute from their initial volume and initial concentration. Use total volume as #0.034" L"#.

#color(orange)"Acetic acid:"#

  • #(0.010" L") * (1.0" M") = 0.01" moles"#

  • #(0.01" moles")/(0.034" L") = 0.29" M"#

#color(green)"Sodium acetate:"#

  • #(0.024" L") * (1.0" M") = 0.024" moles"#

  • #(0.024" moles")/(0.034" L") =0.71" M"#

#"Step 2"#
Look up the #K_(a)# of #"acetic acid"# from your chemistry book or the Web.
#color(white)(aaaaaaaaaaaaaaa)##K_(a) = 1.8 * 10^-5#
(source:preparatorychemistry.com/Bishop_weak_acid_Equilibrium.htm)

To find the #pKa# of acetic acid, take the negative log of the #K_(a)#

  • #pKa = -log(1.8*10^-5)#
  • #pKa = 4.74#

#"Step 3"#
Plugin and solve

  • #pH = pKa + log[("conjugate base")/("acid")]#

  • #pH = 4.74 + log[(0.71 cancelM)/(0.29 cancelM")]#

  • #pH = 4.74 + log[2.45]#

  • #pH = 4.74 + 0.39#

  • #pH = 5.13#

You can check your answer by seeing that you clearly have more than #"2x"# the amount of the #"conjugate base"# than the #"weak acid"# so your answer should reflect a higher #"pH"#.