I'll show you hot to solve this one without using the Henderson - Hasselbalch equation. This method is a bit long, but I think that it's a great help to understanding the general idea behind how buffers work.
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So, your solution is said to contain #""HA""#, a weak acid, and #""NaA""#, the salt of its conjugate base, the #""A""^(-)# anion.
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You know that this buffer is prepared by dissolving #""1 mole""# of each chemical species in #""1.0 L""# of distilled water. Use this information to find the molarity of the weak acid and of the conjugate base
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+#color(blue)(c = n/V)#
+#[""HA""] = [""A""^(-)] = ""1.0 moles""/""1.0 L"" = ""1.0 M""#
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Keep in mind that the salt dissociates in a #1:1# mole ratio to form sodium cations, #""Na""^(+)#, and the conjugate base of the acid, #""A""^(-)#.
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Now, you take #""500 mL""# of this buffer solution. Calculate how many moles of weak acid and conjugate base will be present in this sample
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+#color(blue)(c = n/V implies n = c * V)#
+#n_(HA) = n_(A^(-)) = ""1.0 M"" * 500 * 10^(-3)""L"" = ""0.50 moles""#
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To find the pH of this buffer solution, you need to use an ICE table - remember to use the molarities of the weak acid and conjugate base!
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+#"" """"HA""_text((aq]) + ""H""_2""O""_text((l]) "" ""rightleftharpoons"" "" ""H""_3""O""_text((aq])^(+) "" ""+"" "" ""A""_text((aq])^(-)#
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#color(purple)(""I"")"" "" "" ""1.0"" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" ""0"" "" "" "" "" "" "" "" "" "" ""1.0#
+#color(purple)(""C"")"" ""(-x)"" "" "" "" "" "" "" "" "" "" "" "" "" "" "" ""(+x)"" "" "" "" "" "" ""(+x)#
+#color(purple)(""E"")"" ""1.0-x"" "" "" "" "" "" "" "" "" "" "" "" "" "" "" ""x"" "" "" "" "" "" "" "" ""1.0+x#
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By definition, the acid dissociation constant, #K_a#, will be equal to
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+#K_a = ( [""H""_3""O""^(+)] * [""A""^(-)])/([""HA""]) = (x * (1+x))/(1-x) = 1.75 * 10^(-5)#
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Because the value of #K_a# is so small, you can use the approximation
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+#1.0 +- x ~~ 1.0#
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This will get you
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+#1.75 * 10^(-5) = (x * 1)/(1) = x#
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Here #x# represents the equilibrium concentration of the hydronium ions, so
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+#color(blue)(""pH"" = - log([""H""_3""O""^(+)]))#
+#""pH""_0 = - log(1.75 * 10^(-5)) = 4.757#
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SIDE NOTE This is why the pH of a buffer solution is said to be equal to the acid's #pK_a# when equal concentrations of weak acid and conjugate base are present.
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Now, sodium hydroxide is a strong base, which means that it will neutralize the weak acid to produce water and the conjugate base of the acid.
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This means that you can expect the pH of the solution to increase a little after the addition of the strong base.
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The balanced chemical equation for this reaction looks like this - I'll use the hydroxide anion to represent the strong base
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+#""HA""_text((aq]) + ""OH""_text((aq])^(-) -> ""A""_text((aq])^(-) + ""H""_2""O""_text((l])#
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Notice that the hydroxide anions react in a #1:1# mole ratio with the weak acid and produce conjugate base in the same #1:1# mole ratio.
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Use the molarity and volume of the sodium hydroxide solution to find how many moles of strong base are added to the buffer
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+#color(blue)(c = n/V implies n = c * V)#
+#n_(OH^(-)) = ""0.20 M"" * 25.0 * 10^(-3)""L"" = ""0.0050 moles OH""^(-)#
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This means that after the neutralization reaction takes place, you will be left with
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+#n_(OH^(-)) = ""0 moles"" -># completely consumed
+#n_(HA) = ""0.50 moles"" - ""0.0050 moles"" = ""0.495 moles HA""#
+#n_(A^(-)) = ""0.50 moles"" + ""0.0050 moles"" = ""0.505 moles A""^(-)#
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The new volume of the buffer will be
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+#V_""total"" = V_""initial"" + V_""base""#
+#V_""total"" = ""500 mL"" + ""25.0 mL"" = ""525 mL""#
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Now calculate the new molarities of the weak acid and conjugate base
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+#[""HA""] = ""0.495 moles""/(525 * 10^(-3)""L"") = ""0.9429 M""#
+#[""A""^(-)] = ""0.505 moles""/(525 * 10^(-3)""L"") = ""0.9619 M""#
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Now it's back to the ICE table. To keep the answer at a reasonable length, I won't add it again. This time, the initial concentrations of the weak acid and conjugate base will no longer be #""1.0 M""#.
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Once again, you will have
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+#K_a = ([""H""_3""O""^(+)] * [""A""^(-)])/([""HA""])#
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In this case, this will be equal to
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+#K_a = (x * (0.9619 + x))/(0.9429 + x) = 1.75 * 10^(-5)#
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Using the same approximation, you can say that
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+#1.75 * 10^(-5) = x * 0.9619/0.9429#
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Therefore,
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+#x = (1.75 * 0.9429)/0.9619 * 10^(-5) = 1.7154 * 10^(-5)#
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The pH of the solution will now be
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+#""pH""_1 = - log(1.7154 * 10^(-5)) = 4.766#
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The change in pH can be calculated as
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+#Delta_""pH"" = ""pH""_1 - ""pH""_0#
+#Delta_""pH"" = 4.766 - 4.757 = color(green)(0.009)#
+