Sergey Bylikin, Gary Horner and, Brian Murphy Solutions for Chapter: Medicinal Chemistry, Exercise 4: Questions

An acetate buffer was prepared from $500 {\mathrm{cm}}^3$ of $0.100 \mathrm{mol} {\mathrm{dm}}^{-3}$ ethanoic acid (${\mathrm{pK}}_{\mathrm{a}}=4.76$) and $16.4 \mathrm{g}$ of solid sodium acetate. Deduce the molecular and ionic equations that show the buffer action of this solution when a small amount of hydrochloric acid is added and when a small amount of sodium hydroxide is added.

Phosphoric acid (${\mathrm{pK}}_{{\mathrm{a}}_1}=2.12, {\mathrm{pK}}_{{\mathrm{a}}_2}=7.20, {\mathrm{pK}}_{{\mathrm{a}}_3}=12.3$) and its ions can produce several acid-base buffer systems that exist at different $\mathrm{pH}$. Identify the conjugate acid and the conjugate base in the buffer solution with $\mathrm{pH}=6.8$ prepared from phosphoric acid and sodium hydroxide. 

Phosphoric acid (${\mathrm{pK}}_{{\mathrm{a}}_1}=2.12, {\mathrm{pK}}_{{\mathrm{a}}_2}=7.20, {\mathrm{pK}}_{{\mathrm{a}}_3}=12.3$) and its ions can produce several acid-base buffer systems that exist at different $\mathrm{pH}$. Calculate the mole ratio of the conjugate acid (${\mathrm{H}}_2{{\mathrm{PO}}_4}^{-}$) and conjugate base (${{\mathrm{HPO}}_4}^{2-}$) at $\mathrm{pH}$ $6.8$ in the solution.

Phosphoric acid (${\mathrm{pK}}_{{\mathrm{a}}_1}=2.12, {\mathrm{pK}}_{{\mathrm{a}}_2}=7.20, {\mathrm{pK}}_{{\mathrm{a}}_3}=12.3$) and its ions can produce several acid-base buffer systems that exist at different $\mathrm{pH}$. Deduce molecular and ionic equations that show the buffer action of this solution.

Phosphoric acid (${\mathrm{pK}}_{{\mathrm{a}}_1}=2.12, {\mathrm{pK}}_{{\mathrm{a}}_2}=7.20, {\mathrm{pK}}_{{\mathrm{a}}_3}=12.3$) and its ions can produce several acid-base buffer systems that exist at different $\mathrm{pH}$. Suggest how the mole ratio of conjugate base to the conjugate acid in the buffer solution with $\mathrm{pH}=6.8$ prepared from phosphoric acid and sodium hydroxide will change, when the buffer solution is diluted with an equal volume of water.

An ammonia buffer with $\mathrm{pH}=8.8$ was prepared by dissolving solid ammonium chloride in $0.100 {\mathrm{dm}}^3$ of a $0.200 \mathrm{mol} {\mathrm{dm}}^{-3}$ solution of ammonia. The ${\mathrm{pK}}_{\mathrm{a}}$ for ammonium ion is $9.25$. Calculate the mass of solid ammonium chloride that was used to prepare this solution. Assume that the solution volume did not change when ammonium chloride was added.

Calculate the volumes, in ${\mathrm{cm}}^3$ of $0.100 \mathrm{mol} {\mathrm{dm}}^{-3}$ solutions of sodium carbonate and sodium hydrogencarbonate that need to be mixed together to prepare $300 {\mathrm{cm}}^3$ of a buffer solution with $\mathrm{pH} 10.0$. The ${\mathrm{pK}}_{{\mathrm{a}}_2}$for carbonic acid is $10.3$. Assume that the volume of the final solution is equal to the sum of volumes of initial solutions.

$100 {\mathrm{cm}}^3$ of  $0.100 \mathrm{mol} {\mathrm{dm}}^{-3}$ solutions of sodium carbonate and $200 {\mathrm{cm}}^3$ of $0.100 \mathrm{mol} {\mathrm{dm}}^{-3}$ sodium hydrogencarbonate are mixed together to prepare $300 {\mathrm{cm}}^3$ of a buffer solution with $\mathrm{pH} 10.0$. The ${\mathrm{pK}}_{{\mathrm{a}}_2}$for carbonic acid is $10.3$. This buffer solution is mixed with $50.0 {\mathrm{cm}}^3$ of $10.0 \mathrm{mmol} {\mathrm{dm}}^{-3}$ hydrochloric acid. Calculate the $\mathrm{pH}$ of the final solution. Assume that the volume of the final solution is equal to the sum of volumes of initial solutions.