The conjugate base of H2PO4^- is
(a) PO4^{3-}
(b) P2O5
(c) H3PO4
(d) HPO4^{2-}

Q: What exactly is a conjugate base?

According to the Bronsted-Lowry theory, when an acid donates a proton ($\text{H}^+$), the species that remains is called its conjugate base. It's "conjugate" because it can potentially accept a proton back to reform the acid.

Q: Why does the charge change from $-1$ to $-2$?

You are removing a positively charged proton ($\text{H}^+$). Mathematically, subtracting $+1$ from $-1$ gives you $-2$. The loss of a positive charge leaves the molecule more negative.

Q: Can $\text{H}_2\text{PO}_4^-$ act as a base instead of an acid?

Yes! It is an amphoteric species. If it acts as a base, it will accept a proton ($\text{H}^+$) to form its conjugate acid, which is phosphoric acid ($\text{H}_3\text{PO}_4$).

Q: What happens if we remove another proton from $\text{HPO}_4^{2-}$?

If $\text{HPO}_4^{2-}$ acts as an acid and loses another proton, it forms the phosphate ion ($\text{PO}_4^{3-}$), which is its conjugate base.

Q: Why doesn't it lose all its protons at once?

Phosphoric acid ($\text{H}_3\text{PO}_4$) is a weak polyprotic acid. It loses its protons one by one in successive equilibrium steps, and each subsequent proton is harder to remove because the species becomes increasingly negatively charged.

Question

The conjugate base of $	ext{H}_2	ext{PO}_4^-$ is

Options:

(A) $	ext{PO}_4^{3-}$

(B) $	ext{P}_2	ext{O}_5$

(C) $	ext{H}_3	ext{PO}_4$

(D) $	ext{HPO}_4^{2-}$

Sigma Percentile · Accepted Answer

### The Bronsted-Lowry Perspective

To master acid-base chemistry, we must first look through the lens of the **Bronsted-Lowry theory**. In this framework, acids and bases are defined by a simple transaction: the transfer of a proton ($	ext{H}^+$). An acid is a generous species that donates a proton, while a base is the species that accepts it.

When an acid successfully gives away its proton, it doesn't just disappear. The remnant of that molecule is a new chemical species, which we call the **conjugate base**. It is termed "conjugate" because the reaction is reversible; this new base could, in theory, accept a proton to become the original acid again.

### The Master Rule for Conjugate Bases

Finding the conjugate base of any given acid is one of the most straightforward and satisfying operations in chemistry. You only need to follow one master rule:

$$	ext{Conjugate Base} = 	ext{Acid} - 	ext{H}^+$$

This means you must do two things simultaneously. First, subtract exactly one hydrogen atom from the chemical formula. Second, subtract $+1$ from the overall charge of the molecule. Subtracting a positive charge is mathematically identical to adding a negative charge.

### Applying the Rule to Dihydrogen Phosphate

Let's apply our master rule to the molecule in question: the dihydrogen phosphate ion, **$	ext{H}_2	ext{PO}_4^-$**.

We start by removing one hydrogen atom. The formula goes from having two hydrogens ($	ext{H}_2$) to just one ($	ext{H}$), leaving us with the skeleton $	ext{HPO}_4$.

Next, we adjust the charge. The original ion has a charge of $-1$. When we remove the positively charged proton ($	ext{H}^+$), the remaining species becomes even more negative. Mathematically, $-1 - (+1) = -2$. Therefore, the new charge is $-2$.

Combining these two steps, we get our final answer:

$$	ext{H}_2	ext{PO}_4^- ightleftharpoons 	ext{H}^+ + 	ext{HPO}_4^{2-}$$

The conjugate base is **$	ext{HPO}_4^{2-}$**, known as the hydrogen phosphate ion.

### The Amphoteric Nature

It is fascinating to note that $	ext{H}_2	ext{PO}_4^-$ is an **amphoteric** species. This means it has a dual personality. While we just saw it act as an acid by donating a proton, it can also act as a base! If it were to accept a proton, it would form its conjugate acid, phosphoric acid ($	ext{H}_3	ext{PO}_4$). Understanding this dual nature is key to mastering polyprotic acids in ionic equilibrium.

Animated Solution for Chemistry - Ionic Equilibrium: The conjugate base of $H_{2} PO_{4}^{-}$ is

Select Answer:

Visualized Solution

$Concept of Conjugate Base$

$The Master Rule$

$Applying the Rule$

$Final Answer$

$The Way Forward$

The Sigma Insight: Acid Base Concepts

The Bronsted-Lowry Perspective

The Master Rule for Conjugate Bases

Applying the Rule to Dihydrogen Phosphate

The Amphoteric Nature

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Animated Solution for Chemistry - Ionic Equilibrium: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The conjugate base of H2​PO4−​ is