Question

The known per-oxoacids of Sulphur are :
A. $H_2SO_5$
B. $H_2S_2O_9$
C. $H_3S_4O_{12}$
D. $H_2S_2O_8$
E. $H_2S_2O_7$
Choose the correct answer from the options given below :

• A. A, C only
• B. A, D only
• C. A, B, C only
• D. C, D, E only

Hint:

The oxidation state trick is powerful: Assign H as +1 and O as -2. If the central atom's calculated oxidation state exceeds its group valency maximum (like $> +6$ for S, or $> +5$ for P), the molecule almost certainly contains a peroxide ($-O-O-$) linkage!

Answer 1

The Correct Option is B

Step 1: Concept:
The question asks to identify the specific per-oxoacids (also called peroxy acids) among a list of sulfur-containing oxoacids. A per-oxoacid must contain a peroxide linkage ($-O-O-$ bond).

Step 2: Key Formula or Approach:
One quick way to identify the potential for a peroxide linkage is to calculate the formal oxidation state of the central atom (Sulfur) assuming all oxygens are $-2$. Sulfur's maximum oxidation state is $+6$ (Group 16). If the calculated oxidation state exceeds $+6$, the molecule must contain a peroxide bond (where the oxygen atoms are in a $-1$ oxidation state) to maintain chemical realism.

Step 3: Step-by-step Explanation:

• A. $H_2SO_5$: Calculating S oxidation state: $2(+1) + S + 5(-2) = 0 \implies S = +8$. Since $+8 > +6$, there must be a peroxide linkage. This is structurally known as peroxomonosulfuric acid, or Caro's acid. It has one $-O-O-H$ group. (Yes)

• B. $H_2S_2O_9$: Calculating S oxidation state: $2(+1) + 2S + 9(-2) = 0 \implies 2S = +16 \implies S = +8$. While mathematically it suggests peroxide, $H_2S_2O_9$ is not a standard, well-known stable oxoacid taught in main group chemistry compared to the others.

• C. $H_3S_4O_{12$:} This is not a recognized, stable standard oxoacid formula for sulfur.

• D. $H_2S_2O_8$: Calculating S oxidation state: $2(+1) + 2S + 8(-2) = 0 \implies 2S = 14 \implies S = +7$. Since $+7 > +6$, there is a peroxide linkage. This is peroxodisulfuric acid, or Marshall's acid. It features a central $-O-O-$ bridge connecting two $SO_3H$ groups. (Yes)

• E. $H_2S_2O_7$: Calculating S oxidation state: $2(+1) + 2S + 7(-2) = 0 \implies 2S = 12 \implies S = +6$. The oxidation state is exactly $+6$, the maximum normal state. There is no peroxide linkage. This is pyrosulfuric acid (oleum), which has a standard $-O-$ bridge linking the two sulfur atoms. (No)

• The well-known, textbook per-oxoacids of sulfur are indeed Caro's acid ($H_2SO_5$) and Marshall's acid ($H_2S_2O_8$).

Step 4: Final Answer:
The recognized per-oxoacids are A and D. This corresponds to option (B).