Chapter 14

Calculate the pH of a solution prepared by mixing $2.00\mathrm{g}$ of butyric acid $\left({\mathrm{HC}}_4{\mathrm{H}}_7{\mathrm{O}}_2\right)$ with $0.50\mathrm{g}$ of $\mathrm{NaOH}$ in water $(K_{\mathrm{a}}$ butyric acid $=1.5\times {10}^{-5})$

Calculate the pH of a solution prepared by mixing $100.0\mathrm{mL}$ of $1.20\mathrm{M}$ ethanolamine, ${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{ONH}}_2$, with $50.0\mathrm{mL}$ of $1.0\mathrm{M}\mathrm{HCl}.{\mathrm{K}}_{\mathrm{a}}$ for ${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{ONH}}_3+$ is $3.6\times {10}^{-10}$

A sodium hydrogen carbonate-sodium carbonate buffer is to be prepared with a $\mathrm{pH}$ of $9.40$. (a) What must the $\left[{\mathrm{HCO}}_3^{-}\right]/\left[{\mathrm{CO}}_3^{2-}\right]$ ratio be? (b) How many moles of sodium hydrogen carbonate must be added to a liter of $0.225\mathrm{M}{\mathrm{Na}}_2{\mathrm{CO}}_3$ to give this $\mathrm{pH}$ ? (c) How many grams of sodium carbonate must be added to $475\mathrm{mL}$ of $0.336\mathrm{M}{\mathrm{NaHCO}}_3$ to give this $\mathrm{pH}$ ? (Assume no volume change.) (d) What volume of $0.200\mathrm{M}{\mathrm{NaHCO}}_3$ must be added to $735\mathrm{mL}$ of a $0.139\mathrm{M}$ solution of ${\mathrm{Na}}_2{\mathrm{CO}}_3$ to give this $\mathrm{pH}$ ? (Assume that volumes are additive.)

WEB To make a buffer with $\mathrm{pH}=3.0$ from ${\mathrm{HCHO}}_2$ and ${\mathrm{CHO}}_2^{-}$, (a) what must the [HCHO ${}_2]/\left[{\mathrm{CHO}}_2^{-}\right]$ ratio be? (b) how many moles of ${\mathrm{HCHO}}_2$ must be added to a liter of $0.139\mathrm{M}$ ${\mathrm{NaCHO}}_2$ to give this $\mathrm{pH}$ ? (c) how many grams of ${\mathrm{NaCHO}}_2$ must be added to $350.0\mathrm{mL}$ of $0.159{\mathrm{MHCHO}}_2$ to give this $\mathrm{pH}$ ? (d) What volume of $0.236\mathrm{M}{\mathrm{HCHO}}_2$ must be added to $1.00\mathrm{L}$ of a $0.500\mathrm{M}$ solution of ${\mathrm{NaCHO}}_2$ to give this pH? (Assume that volumes are additive.)

A buffer is made up of $0.300$ L each of $0.500{\mathrm{MKH}}_2{\mathrm{PO}}_4$ and $0.317\mathrm{M}$ ${\mathrm{K}}_2{\mathrm{HPO}}_4$. Assuming that volumes are additive, calculate (a) the $\mathrm{pH}$ of the buffer. (b) the $\mathrm{pH}$ of the buffer after the addition of $0.0500\mathrm{mol}$ of $\mathrm{HCl}$ to $0.600\mathrm{L}$ of buffer. (c) the $\mathrm{pH}$ of the buffer after the addition of $0.0500\mathrm{mol}$ of $\mathrm{NaOH}$ to $0.600\mathrm{L}$ of buffer.

A buffer is made up of $355\mathrm{mL}$ each of $0.200\mathrm{M}{\mathrm{NaHCO}}_3$ and $0.134\mathrm{M}$ ${\mathrm{Na}}_2{\mathrm{CO}}_3$. Assuming that volumes are additive, calculate (a) the $\mathrm{pH}$ of the buffer. (b) the $\mathrm{pH}$ of the buffer after the addition of $0.0300\mathrm{mol}$ of $\mathrm{HCl}$ to $0.710\mathrm{L}$ of buffer. (c) the $\mathrm{pH}$ of the buffer after the addition of $0.0300\mathrm{mol}$ of $\mathrm{KOH}$ to $0.710\mathrm{L}$ of buffer.

A buffer is prepared using the propionic acid/propionate $({\mathrm{HC}}_3{\mathrm{H}}_5{\mathrm{O}}_2/$ ${\mathrm{C}}_3{\mathrm{H}}_5{\mathrm{O}}_2^{-})$ acid-base pair for which the ratio $\left[{\mathrm{HC}}_3{\mathrm{H}}_5{\mathrm{O}}_2\right]/\left[{\mathrm{C}}_3{\mathrm{H}}_5{\mathrm{O}}_2^{-}\right]$ is $4.50.$ $K_{\mathrm{a}}$ for propionic acid is $1.4\times {10}^{-5}$. (a) What is the $\mathrm{pH}$ of this buffer? (b) Enough strong base is added to convert $27\mathrm{\%}$ of ${\mathrm{HC}}_3{\mathrm{H}}_5{\mathrm{O}}_2$ to ${\mathrm{C}}_3{\mathrm{H}}_5{\mathrm{O}}_2^{-}.$ What is the $\mathrm{pH}$ of the resulting solution? (c) Strong base is added to increase the $\mathrm{pH}$. What must the acid/base ratio be so that the $\mathrm{pH}$ increases by exactly one unit (e.g., from 2 to 3 ) from the answer in (a)?

There is a buffer system $({\mathrm{H}}_2{\mathrm{PO}}_4^{-}-{\mathrm{HPO}}_4{}^{2-})$ in blood that helps keep the blood $\mathrm{pH}$ at about $7.40.({\mathrm{K}}_{\mathrm{a}}{\mathrm{H}}_2{\mathrm{PO}}_4^{-}=6.2\times {10}^{-8})$. (a) Calculate the $\left[{\mathrm{H}}_2{\mathrm{PO}}_4^{-}\right]/\left[{\mathrm{HPO}}_4^{2-}\right]$ ratio at the normal $\mathrm{pH}$ of blood. (b) What percentage of the ${\mathrm{HPO}}_4{}^{2-}$ ions are converted to ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$ when the $\mathrm{pH}$ goes down to $6.80$ ? (c) What percentage of ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$ ions are converted to ${\mathrm{HPO}}_4{}^{2-}$ when the $\mathrm{pH}$ goes up to $7.80$ ?

Given three acid-base indicators-methyl orange (end point at $\mathrm{pH}$ 4), bromthymol blue (end point at $\mathrm{pH}7$ ), and phenolphthalein (end point at $\mathrm{pH}$ 9) - which would you select for the following acid-base titrations? (a) perchloric acid with an aqueous solution of ammonia (b) nitrous acid with lithium hydroxide (c) hydrobromic acid with strontium hydroxide (d) sodium fluoride with nitric acid

Metacresol purple is an indicator that changes from yellow to purple at $\mathrm{pH}8.2$. (a) What is $K_{\mathrm{a}}$ for this indicator? (b) What is its $\mathrm{pH}$ range? (c) What is the color of a solution with $\mathrm{pH}9.0$ and a few drops of metacresol purple?