Question

# Identify the most acidic oxide among the following oxides:(A) $\text{S}{{\text{O}}_{3}}$(B) ${{\text{P}}_{4}}{{\text{O}}_{10}}$(C) $\text{C}{{\text{l}}_{2}}{{\text{O}}_{7}}$(D) ${{\text{N}}_{2}}{{\text{O}}_{5}}$

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Hint: The acidic character of oxides depends on the position of the nonmetals in the periodic table and the oxidation state of the non metal.

In the periodic table, nitrogen is present in the second period and group 15. Phosphorus, sulphur and chlorine are present in third period and groups 15,16 and 17 respectively.

On going from left to right in a period, the acidic nature of oxides increases. Hence, among ${{\text{P}}_{4}}{{\text{O}}_{10}},\text{ S}{{\text{O}}_{3}}\text{ and C}{{\text{l}}_{2}}{{\text{O}}_{7}}$, the most acidic oxide is $\text{C}{{\text{l}}_{2}}{{\text{O}}_{7}}$.
On going from top to bottom in a period, the acidic nature of oxides decreases due to decrease in the non metallic character. Hence, among ${{\text{N}}_{2}}{{\text{O}}_{5}}\text{ and }{{\text{P}}_{4}}{{\text{O}}_{10}}$, most acidic oxide is ${{\text{N}}_{2}}{{\text{O}}_{5}}$.
Hence, either ${{\text{N}}_{2}}{{\text{O}}_{5}}$ or $\text{C}{{\text{l}}_{2}}{{\text{O}}_{7}}$ is probably the correct answer.
For non metal oxides, the acidic character increases with increase in the oxidation state. With increase in the oxidation state, the covalent character increases and the polarising power also increases. This increases the acidic character.

The oxidation states of non metals nitrogen, phosphorus, sulphur and chlorine are ${{\text{N}}_{2}}{{\text{O}}_{5}},\text{ }{{\text{P}}_{4}}{{\text{O}}_{10}},\text{ S}{{\text{O}}_{3}}\text{ and C}{{\text{l}}_{2}}{{\text{O}}_{7}}$ are +5, +5, +6 and +7 respectively. Thus, chlorine has the highest oxidation state. Hence, $\text{C}{{\text{l}}_{2}}{{\text{O}}_{7}}$ is the most acidic oxide.

Hence, option C is the correct answer.

Note:
Do not ignore the effect of oxidation state of non metal on the acidic nature of the oxide.For non metal oxides, the acidic character increases with increase in the oxidation state. With increase in the oxidation state, the covalent character increases and the polarising power also increases. This increases the acidic character.
The oxidation state of oxygen is -2. Let X be the oxidation state of non metal.
For, $\text{S}{{\text{O}}_{3}}$ molecule, $X+3\left( -2 \right)=0\text{ }or\text{ }X=+6.$
For ${{\text{P}}_{4}}{{\text{O}}_{10}}$ molecule, $4\left( X \right)+10\left( -2 \right)=0\text{ }or\text{ }4X=20\text{ }or\text{ }X=+5.$
For $\text{C}{{\text{l}}_{2}}{{\text{O}}_{7}}$ molecule, $2\left( X \right)+7\left( -2 \right)=0\text{ }or\text{ }2X=14\text{ }or\text{ }X=+7.$
For ${{\text{N}}_{2}}{{\text{O}}_{5}}$ molecule, $2\left( X \right)+5\left( -2 \right)=0\text{ }or\text{ }2X=10\text{ }or\text{ }X=+5.$