Problem 51 - Chemistry An Atoms First Approach

Question

Phosphoric acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)\) is a triprotic acid, phosphorous acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{3}\right)\) is a diprotic acid, and hypophosphorous acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{2}\right)\) is a monoprotic acid. Explain this phenomenon.

Step-by-Step Solution

Answer

The phenomenon of phosphoric acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)\) being a triprotic acid, phosphorous acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{3}\right)\) being a diprotic acid, and hypophosphorous acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{2}\right)\) being a monoprotic acid is based on their molecular structures. The number of hydrogen atoms bonded to oxygen atoms in each molecule determines how many protons they can donate in a solution. Phosphoric acid has three hydrogen atoms bonded to acidic oxygen atoms, phosphorous acid has two, and hypophosphorous acid has one; hence they are triprotic, diprotic, and monoprotic acids, respectively.

Step 1: Define monoprotic, diprotic, and triprotic acids

Monoprotic acids are substances that can donate one proton (H+) in a solution, diprotic acids can donate two protons, and triprotic acids can donate three protons.

Step 2: Examine phosphoric acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)\) structure

Phosphoric acid has the chemical formula \(\mathrm{H}_{3} \mathrm{PO}_{4}\), which means it has three hydrogen atoms bonded to a central phosphorus atom. The Lewis structure of phosphoric acid shows that all three hydrogen atoms are bonded to oxygen atoms, which are also attached to the central phosphorus atom (P). These hydrogen atoms can be donated as protons in a reaction.

Step 3: Examine phosphorous acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{3}\right)\) structure

Phosphorous acid has the chemical formula \(\mathrm{H}_{3} \mathrm{PO}_{3}\). The Lewis structure reveals that only two of the three hydrogen atoms are bonded to oxygen atoms, which are also attached to the central phosphorus atom. As a result, phosphorous acid can donate only two protons in a reaction.

Step 4: Examine hypophosphorous acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{2}\right)\) structure

Hypophosphorous acid has the chemical formula \(\mathrm{H}_{3} \mathrm{PO}_{2}\). The Lewis structure shows that only one hydrogen atom is bonded to an oxygen atom, which is also attached to the central phosphorus atom. This means hypophosphorous acid can donate only one proton in a reaction.

Step 5: Conclusion

The phenomenon of phosphoric acid being a triprotic acid, phosphorous acid being a diprotic acid, and hypophosphorous acid being a monoprotic acid is based on their molecular structures. The number of hydrogen atoms bonded to oxygen atoms in each molecule determines how many protons they can donate in a solution. Phosphoric acid has three hydrogen atoms bonded to acidic oxygen atoms, phosphorous acid has two, and hypophosphorous acid has one; hence they are triprotic, diprotic, and monoprotic acids, respectively.