Question

Correctly label the speciation diagram below:

 

• A. I = PO$_4^{3-}$, II = HPO$_4^{2-}$, III = H$_2$PO$_4^{-}$, IV = H$_3$PO$_4$
• B. I = H$_3$PO$_4$, II = H$_2$PO$_4^{-}$, III = HPO$_4^{2-}$, IV = PO$_4^{3-}$
• C. I = H$_3$PO$_4$, II = H$_2$PO$_4^{-}$, III = HPO$_4^{2-}$, IV = PO$_4^{3-}$
• D. I = PO$_4^{3-}$, II = H$_2$PO$_4^{-}$, III = H$_3$PO$_4$, IV = HPO$_4^{2-}$

Hint:

In speciation diagrams of polyprotic acids, remember that the most protonated form dominates at low pH, and as pH increases, each subsequent deprotonated form becomes dominant around its corresponding pKa value. The number of curves typically corresponds to the number of dissociable protons plus one.

Answer 1

The Correct Option is C

Step 1: Understand the protonation states of phosphoric acid.
Phosphoric acid (H$_3$PO$_4$) is a triprotic acid. It loses protons stepwise: \[ \mathrm{H_3PO_4} \xrightarrow{-\mathrm{H}^+} \mathrm{H_2PO_4^-} \xrightarrow{-\mathrm{H}^+} \mathrm{HPO_4^{2-}} \xrightarrow{-\mathrm{H}^+} \mathrm{PO_4^{3-}} \] The approximate pKa values for each step are: \( \mathrm{p}K_\mathrm{a1} \approx 2.1 \), \( \mathrm{p}K_\mathrm{a2} \approx 7.2 \), and \( \mathrm{p}K_\mathrm{a3} \approx 12.3 \).

Step 2: Analyze the speciation diagram based on pH.
Curve I: Dominates at low pH (pH < pKa$_1$), so it represents fully protonated H$_3$PO$_4$.
Curve II: Peaks between pKa$_1$ and pKa$_2$, representing H$_2$PO$_4^{-}$.
Curve III: Peaks between pKa$_2$ and pKa$_3$, representing HPO$_4^{2-}$.
Curve IV: Dominates at high pH (pH > pKa$_3$), corresponding to PO$_4^{3-}$.

Step 3: Match the curves with the chemical species.
I = H$_3$PO$_4$
II = H$_2$PO$_4^{-}$
III = HPO$_4^{2-}$
IV = PO$_4^{3-}$

Step 4: Compare with the given options.
Only option (3) matches the above assignments.