Question

The anhydride of pyrosulphuric acid is__________.

• A. ${\mathrm{SO}}_2$
• B. ${\mathrm{SO}}_3$
• C. ${\mathrm{S}}_2{\mathrm{O}}_7^{-2}$
• D. ${\mathrm{S}}_2{\mathrm{O}}_3^{-2}$

Answer 1

The correct option is B

${\mathrm{SO}}_3$

Let the oxidation state of sulfur in Pyrosulphuric Acid$\left({\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_7\right)$ is 'x'.

Then

$$\begin{gathered} 2(+1)+2\text{'}\mathrm{x}\text{'}+7(-2)=0 \\ 2+2\mathrm{x}-14=0 \\ 2\mathrm{x}=12 \\ \mathrm{x}=+6 \end{gathered}$$.

The oxidation state of sulfur in Pyrosulphuric Acid$\left({\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_7\right)$ is '+6'

Then the oxidation state of sulfur in anhydride of Pyrosulphuric Acid$\left({\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_7\right)$ must also be +6.

Explanation of correct option

Option (B)

Let, the oxidation state of sulfur in sulfur trioxide${\mathrm{SO}}_3$ is 'p'.

Then

$$\begin{gathered} \mathrm{p}+3(-2)=0 \\ \mathrm{p}-6=0 \\ \mathrm{p}=+6 \end{gathered}$$

The oxidation state of sulfur in sulfur trioxide${\mathrm{SO}}_3$ is '+6'.

Explanation of Incorrect option

Option (A)

Let the oxidation state of sulfur in sulfur dioxide${\mathrm{SO}}_2$ is ‘q’.

Then

$$\begin{gathered} \mathrm{q}+2(-2)=0 \\ \mathrm{q}-4=0 \\ \mathrm{q}=+4 \end{gathered}$$

Option (C)

Let the oxidation state of sulfur ${\mathrm{S}}_2{\mathrm{O}}_7^{-2}$ is ‘r’.

$$\begin{gathered} 2\left(\mathrm{r}\right)+7(-2)=(-2) \\ 2\mathrm{r}-14=-2 \\ 2\mathrm{r}=+12 \\ \mathrm{r}=+6 \end{gathered}$$

The oxidation state of sulfur ${\mathrm{S}}_2{\mathrm{O}}_7^{-2}$ is ‘+6’ also but it is not an anhydride of Pyrosulphuric Acid$\left({\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_7\right)$. As it contains the same number of oxygen atoms.

Option (D)

Let the oxidation state of sulfur ${\mathrm{S}}_2{\mathrm{O}}_3^{-2}$ is ‘s’.

Then

$$\begin{gathered} 2\left(\mathrm{s}\right)+3(-2)=(-2) \\ 2\mathrm{s}-6=-2 \\ 2\mathrm{s}=+4 \\ \mathrm{s}=+2 \end{gathered}$$

The oxidation state of sulfur ${\mathrm{S}}_2{\mathrm{O}}_3^{-2}$ is ‘+2’.

Hence, Option(B) is correct.