Animated Solution for Chemistry - s and p-Block Elements: The pair in which phosphorus atoms have a formal oxidation state of +3 is

Welcome to a fascinating exploration of phosphorus oxoacids! This problem might look like a simple memory test, but it is actually a beautiful exercise in chemical nomenclature and algebraic deduction.

Our mission is clear: we need to identify the specific pair of acids where the central phosphorus atom proudly exhibits a formal oxidation state of $+3$.

To conquer this, we don't need to memorize every single structure. We just need to rely on the fundamental rules of oxidation states and the chemical formulas hidden within their names.

Let's establish our mathematical toolkit. In most oxoacids, hydrogen is assigned an oxidation state of $+1$, and oxygen is assigned $-2$.

Because these molecules are electrically neutral, the sum of the oxidation states of all atoms must perfectly balance out to zero. Let's denote the unknown oxidation state of phosphorus as $x$.

Our first candidate is Orthophosphorous acid. Its chemical formula is $H_{3}P{O}_3$. Let's set up our algebraic equation:

Simplifying this, we get:

Bingo! Orthophosphorous acid is a perfect match. Any option containing it is a strong contender.

While the math gives us the answer, visualizing the structure gives us the feel of the chemistry.

If we draw the structure of $H_{3}P{O}_3$, we see a central phosphorus atom double-bonded to one oxygen, single-bonded to two $-OH$ groups, and uniquely, single-bonded directly to a hydrogen atom.

This direct $P-H$ bond is the reason why orthophosphorous acid is only a dibasic acid, despite possessing three hydrogen atoms. Only the hydrogens attached to the highly electronegative oxygen atoms are ionizable.

Now, let's examine Pyrophosphorous acid. The prefix "pyro" suggests it is formed by heating two molecules of the ortho acid and eliminating a water molecule. Its formula is $H_4{P}_2{O}_5$. Let's run the numbers:

We have found our second match! Structurally, pyrophosphorous acid features a $P-O-P$ bridge connecting two identical phosphorus centers, both sitting comfortably at a $+3$ oxidation state.

To be absolutely rigorous, let's quickly investigate the other acids mentioned in the options to ensure we haven't missed anything.

Consider Hypophosphoric acid, with the formula $H_4{P}_2{O}_6$. Setting up the equation:

Here, phosphorus is in a $+4$ state, so this acid is disqualified.

Next, let's look at Pyrophosphoric acid, $H_4{P}_2{O}_7$. The calculation yields:

With an oxidation state of $+5$, this acid is also out of the running.

Through systematic calculation and structural verification, we have successfully isolated the correct pair.

Both orthophosphorous acid and pyrophosphorous acid feature phosphorus in the exact $+3$ oxidation state we were hunting for.

As a general rule of thumb, remember that acids ending in the suffix "-ous" typically feature the central atom in a lower oxidation state, while those ending in "-ic" feature it in a higher oxidation state. Keep this chemical intuition sharp, and you'll breeze through similar problems!