Acids

Introduced to this in AP Chemistry

Refer to Acid-Base Theories if needed

Example of a strong acid solution §

What is the pH of a 0.50 M solution of HCl

HCl $\to$ H${}^{+}$ + Cl${}^{-}$

0.50 M -> 0.5 M & 0.50 M

pH of the solution is based on the concentration of H${}^{+}$

pH = -log[H${}^{+}$] = -log(0.50) = 0.30

Example of a weak acid solution §

What is the pH of a 0.50 M solution of acetic acid, HC${}_2$H${}_3$O${}_2$, K${}_a$ = 1.8 * 10${}^{-5}$

Step # 1: Write the dissociation equation

HC${}_2$H${}_3$O${}_2$ $\leftrightarrow$ C${}_2$H${}_3$O${}_2^{-}$ + H${}^{+}$

Step # 2: Write the equilibrium expression

K${}_a$ = $\frac{[H{]}^{+}[C_2{H}_3{O}_2{]}^{-}}{[H{C}_2{H}_3{O}_2]}$

Step # 3: ICE it

Step # 5.5: Check 5% rule for approximation

HC${}_2$H${}_3$O${}_2$ $\leftrightarrow$ C${}_2$H${}_3$O${}_2^{-}$ + H${}^{+}$

A Weak Acid Equilibrium Problem §

What is the pH of a 0.25 M solution of carbonic acid

H${}_2$CO${}_3$ $\leftrightarrow$ H${}^{+}$ + HCO${}_3^{-}$ Ka${}_1$

HCO${}_3^{-}$ $\leftrightarrow$ H${}^{+}$ + CO${}_3^{2-}$ Ka${}_2$

Step # 1: Write the dissociation equation

H${}_2$CO${}_3$ $\leftrightarrow$ H${}^{+}$ + HCO${}_3^{-}$ | Ka${}_1$

Step # 2: Write the equilibrium expression

K${}_a$ 1 = $\frac{[H{]}^{+}[HC{O}_3^{]}-}{[H_{2}C{O}_3]}$

Step # 3: ICE it