Question

Phosphoric acid, H₃PO₄, is a triprotic acid. What are the three acids? K_a2 for phosphoric acid is 6.2 × 10^-8. What reaction refers to K_a2?

Short answer

Three acids present in H₃PO₄are

1. H₃PO₄
2. H₂PO₄^-
3. HPO₄^2-

Reaction that refers to K_a2 : ${\text{H}}_{\text{2}}{\text{PO}}_{\text{4}}{{}^{\text{-}}}_{\left(\mathrm{aq}\right)}\rightleftarrows {{\text{\hspace{0.17em}\hspace{1em}H}}^{\text{+}}}_{\left(\mathrm{aq}\right)}{\text{+\hspace{0.17em}HPO}}_{\text{4}}{{}^{\text{2-}}}_{\left(\mathrm{aq}\right)}$

Step-by-step solution

Polyprotic Acids 

The acids which have more than one H⁺ ions to donate are known as polyprotic acid. H₃PO₄ is a triprotic acid as it can donate three H⁺ ions.\

Three acids present in H₃PO₄are

1. H₃PO₄
2. H₂PO₄^-
3. HPO₄^2-

Equation for Ka2

As after giving one proton acid formed is H₂PO₄^- , and its dissociation is represented as

${\text{H}}_{\text{2}}{\text{PO}}_{\text{4}}{{}^{\text{-}}}_{\left(\text{aq}\right)}\rightleftarrows {{\text{\hspace{0.17em}\hspace{1em}H}}^{\text{+}}}_{\left(\text{aq}\right)}{\text{+\hspace{0.17em}HPO}}_{\text{4}}{{}^{\text{2-}}}_{\left(\text{aq}\right)}$

${\text{K}}_{{\mathrm{a}}_{\text{2}}}=\frac{{\text{[H}}^{\text{+}}{{\text{][HPO}}_{\text{4}}}^{\text{2-}}\text{]}}{{\text{[H}}_{\text{2}}{{\text{PO}}_{\text{4}}}^{\text{-}}\text{]}}$