Chapter 8: Applications of Aqueous Equilibria

Calculate the pH of a solution that is $\mathbf{0}\mathbf{.}\mathbf{40}\mathit{M}{\mathit{H}}_{\mathbf{2}}\mathit{N}\mathit{N}{\mathit{H}}_{\mathbf{2}}\mathit{a}\mathit{n}\mathit{d}\mathbf{0}\mathbf{.}\mathbf{80}\mathit{M}{\mathit{H}}_{\mathbf{2}}\mathit{N}\mathit{N}{\mathit{H}}_{\mathbf{3}}\mathit{N}{\mathit{O}}_{\mathbf{3}}\mathbf{.}$In order for this buffer to have $\mathit{p}\mathit{H}\mathbf{=}\mathit{p}{\mathit{K}}_{\mathbf{a}}$ would you add $\mathit{H}\mathit{C}\mathit{l}\mathit{o}\mathit{r}\mathit{N}\mathit{a}\mathit{O}\mathit{H}\mathbf{?}$what quantity of which reagent would you add to $\mathbf{1}\mathbf{.}\mathbf{0}\mathit{L}$of the original buffer so that the resulting solution has$\mathit{p}\mathit{H}\mathbf{=}\mathit{p}{\mathit{K}}_{\mathbf{a}}\mathbf{?}$

Calculate the pH of a solution that is $\mathbf{0}\mathbf{.}\mathbf{20}\mathit{M}\mathit{H}\mathit{O}\mathit{C}\mathit{l}$and $\mathbf{0}\mathbf{.}\mathbf{90}\mathit{M}\mathit{K}\mathit{O}\mathit{C}\mathit{l}$. In order for this buffer to have$\mathit{p}\mathit{H}\mathbf{=}\mathit{p}{\mathit{K}}_{\mathbf{a}}$ would you add $\mathbf{\text{HCl\hspace{0.17em}or\hspace{0.17em}NaOH?}}$what quantity (moles) of which reagent would you add to of the original buffer so that the resulting solution has$\mathit{p}\mathit{H}\mathbf{=}\mathit{p}{\mathit{K}}_{\mathbf{a}}\mathbf{?}$

Calculate the pH of a buffered solution prepared by dissolving$\mathbf{21}\mathbf{.}\mathbf{5}\mathit{g}$ of benzoic acid $\mathit{H}{\mathit{C}}_{\mathbf{7}}{\mathit{H}}_{\mathbf{5}}{\mathit{O}}_{\mathbf{2}}$and $\mathbf{37}\mathbf{.}\mathbf{7}\mathit{g}$of sodium benzoate in$\mathbf{200}\mathbf{.}\mathbf{0}\mathbf{ }\mathit{m}\mathit{L}$ of solution

A buffered solution is made by adding$\mathbf{50}\mathbf{.}\mathbf{0}\mathbf{ }\mathit{g}\mathbf{ }\mathit{N}{\mathit{H}}_{\mathbf{4}}\mathit{C}\mathit{l}\mathbf{ }\mathit{t}\mathit{o}\mathbf{ }\mathbf{1}\mathbf{.}\mathbf{00}\mathbf{ }\mathit{L}\mathbf{ }\mathit{o}\mathit{f}\mathbf{ }\mathit{a}\mathbf{ }\mathbf{0}\mathbf{.}\mathbf{75}\mathbf{ }\mathit{M}\mathbf{ }\mathit{s}\mathit{o}\mathit{l}\mathit{u}\mathit{t}\mathit{i}\mathit{o}\mathit{n}\mathbf{ }\mathit{o}\mathit{f}\mathbf{ }\mathit{N}{\mathit{H}}_{\mathbf{3}}$. Calculate the pH of the final solution. (Assume no volume change)

Consider a solution that contains both${\mathit{C}}_{\mathbf{5}}{\mathit{H}}_{\mathbf{5}}\mathit{N}\mathbf{ }\mathit{a}\mathit{n}\mathit{d}\mathbf{ }{\mathit{C}}_{\mathbf{5}}{\mathit{H}}_{\mathbf{5}}\mathit{N}\mathit{H}\mathit{N}{\mathit{O}}_{\mathbf{3}}\mathbf{.}$ Calculate the ratio$\left[C_5{H}_{5}N\right]\mathbf{/}\left[C_5{H}_{5}N{H}^{+}\right]$if the solution has the following pH values

a)$\mathit{p}\mathit{H}\mathbf{}\mathbf{=}\mathbf{}\mathbf{4}\mathbf{.}\mathbf{50}$

b)$\mathit{p}\mathit{H}\mathbf{}\mathbf{=}\mathbf{}\mathbf{5}\mathbf{.}\mathbf{00}$

c)$\mathit{p}\mathit{H}\mathbf{}\mathbf{=}\mathbf{}\mathbf{5}\mathbf{.}\mathbf{23}$

d)$\mathit{p}\mathit{H}\mathbf{}\mathbf{=}\mathbf{}\mathbf{5}\mathbf{.}\mathbf{50}$

How many moles of $\mathit{N}\mathit{a}\mathit{O}\mathit{H}$must be added to$\mathbf{1}\mathbf{.}\mathbf{0}\mathbf{ }\mathit{L}\mathbf{ }\mathit{o}\mathit{f}\mathbf{ }\mathbf{2}\mathbf{.}\mathbf{0}\mathbf{ }\mathit{M}\mathbf{ }\mathit{H}{\mathit{C}}_{\mathbf{2}}{\mathit{H}}_{\mathbf{3}}{\mathit{O}}_{\mathbf{2}}$to produce a solution buffered at each pH?

a)$\mathbf{\text{pH}}\mathbf{=}{\mathbf{\text{pK}}}_{\mathbf{\text{a}}}$

b)$\mathbf{\text{pH}}\mathbf{=}\mathbf{4}\mathbf{.}\mathbf{00}$

c)$\mathbf{\text{pH}}\mathbf{=}\mathbf{5}\mathbf{.}\mathbf{00}$

Calculate the number of moles of$\mathit{H}\mathit{C}\mathit{l}\mathbf{\left(}\mathit{g}\mathbf{\right)}$must be added to$\mathbf{1}\mathbf{.}\mathbf{0}\mathbf{ }\mathit{L}\mathbf{ }\mathit{o}\mathit{f}\mathbf{ }\mathbf{1}\mathbf{.}\mathbf{0}\mathbf{ }\mathit{M}\mathbf{ }\mathit{N}\mathit{a}{\mathit{C}}_{\mathbf{2}}{\mathit{H}}_{\mathbf{3}}{\mathit{O}}_{\mathbf{2}}$to produce a solution buffered at each pH?

a)$\mathbf{\text{pH}}\mathbf{=}{\mathbf{\text{pK}}}_{\mathbf{\text{a}}}$

b)$\mathbf{\text{pH}}\mathbf{=}\mathbf{4}\mathbf{.}\mathbf{20}$

c) $\mathbf{\text{pH}}\mathbf{=}\mathbf{5}\mathbf{.}\mathbf{00}$

You make of $\mathbf{1}\mathbf{.}\mathbf{00}\mathbf{ }\mathit{L}$a buffered solution by mixing acetic acid and sodium acetate. You have$\mathbf{1}\mathbf{.}\mathbf{00}\mathbf{ }\mathit{M}$ solutions for each component of the buffered solution. What volume of each solution do you mix to make such a buffered solution?

Calculate the mass of sodium acetate that must be added to 500.0 mL of 0.200 M acetic acid to form a pH 5 5.00 buffered solution.

Question: What if all you know about two salts is that the value of K_sp for salt A is greater than that of salt B? Why can we not compare the relative solubilities of the salts? Use numbers to show how salt A could be more soluble than salt B, and how salt B can be more soluble than salt A.