Consider the following reactions :
NaCl + K2Cr2O7 + H2SO4 - (A) + side products (conc)
(A) + NaOH -

Q: Why do we use concentrated $H_2SO_4$ and not dilute?

Concentrated $H_2SO_4$ acts as a strong dehydrating agent and provides the highly acidic medium required to generate the $CrO_2Cl_2$ vapors. Dilute acid won't work because the reaction requires the removal of water to drive it forward.

Q: Can we use this test for bromides and iodides?

No! Bromides and iodides are strong reducing agents. Instead of forming volatile compounds like chromyl chloride, they get oxidized by the dichromate to free bromine ($Br_2$) and iodine ($I_2$) gases.

Q: What is the oxidation state of Chromium in $CrO_5$? Is it +10?

A classic trap! The oxidation state of Cr cannot exceed its group number (+6). In $CrO_5$, there are two peroxo linkages (O-O) where oxygen is in a -1 state, and one double-bonded oxygen in a -2 state. Calculating it: $x + 4(-1) + 1(-2) = 0 \implies x = +6$.

Q: Why does the blue color of $CrO_5$ fade away quickly?

$CrO_5$ is highly unstable in aqueous solutions. It rapidly decomposes to form stable $Cr^{3+}$ ions and oxygen gas, causing the beautiful blue color to fade into a pale green.

Q: What is the geometry of the Chromyl Chloride ($CrO_2Cl_2$) molecule?

It has a tetrahedral geometry. The chromium atom is $sp^3$ hybridized, bonded to two oxygen atoms via double bonds and two chlorine atoms via single bonds.

Question

Consider the following reactions :
$NaCl + K_2Cr_2O_7 + H_2SO_4 ightarrow (A) + 	ext{side products (conc)}$
$(A) + NaOH ightarrow (B) + 	ext{side products}$
$(B) + H_2SO_4 + H_2O_2 ightarrow (C) + 	ext{side products (dilute)}$
The sum of the total number of atoms in one molecule each of (A), (B) and (C) is .............

Sigma Percentile · Accepted Answer

The world of inorganic chemistry is filled with vibrant colors and fascinating molecular transformations. This problem takes us on a thrilling journey through one of the most iconic sequences in qualitative analysis: the Chromyl Chloride test and its subsequent reactions. Our mission is to identify three distinct chromium compounds and count their atoms. Let's dive into the chemistry!

### The Chromyl Chloride Test: A Classic Encounter

The first reaction is the cornerstone of detecting chloride ions. When a solid chloride salt, like sodium chloride ($NaCl$), is heated with potassium dichromate ($K_2Cr_2O_7$) and concentrated sulfuric acid ($H_2SO_4$), a vigorous reaction occurs. The concentrated acid acts as a powerful dehydrating agent and provides the necessary acidic medium.

This reaction produces deep, blood-red vapors of a volatile compound. This compound is **Chromyl Chloride** ($CrO_2Cl_2$), which is our compound **(A)**.

The balanced chemical equation is:
$$4NaCl + K_2Cr_2O_7 + 6H_2SO_4 ightarrow 2CrO_2Cl_2 + 4NaHSO_4 + 2KHSO_4 + 3H_2O$$

Now, let's count the atoms in one molecule of $CrO_2Cl_2$. We have 1 Chromium atom, 2 Oxygen atoms, and 2 Chlorine atoms. 
Total atoms in (A) = $1 + 2 + 2 = 5$.

### The Yellow Transformation: Forming Chromate

In the second step, the red vapors of chromyl chloride are passed through an aqueous solution of sodium hydroxide ($NaOH$). Chromyl chloride is an acidic oxide, and it readily dissolves in the alkali to undergo hydrolysis.

This reaction yields a bright yellow solution, which is characteristic of the **Chromate ion** ($CrO_4^{2-}$). Specifically, we form Sodium Chromate ($Na_2CrO_4$), which is our compound **(B)**.

The balanced chemical equation is:
$$CrO_2Cl_2 + 4NaOH ightarrow Na_2CrO_4 + 2NaCl + 2H_2O$$

Let's count the atoms in one molecule of $Na_2CrO_4$. We have 2 Sodium atoms, 1 Chromium atom, and 4 Oxygen atoms.
Total atoms in (B) = $2 + 1 + 4 = 7$.

### The Blue Butterfly: Chromium Pentoxide

The final reaction is perhaps the most visually stunning. The yellow sodium chromate solution is acidified with dilute sulfuric acid and then treated with hydrogen peroxide ($H_2O_2$).

The peroxide oxidizes the chromate to form a highly unstable, deep blue compound known as **Chromium Pentoxide** ($CrO_5$), which is our compound **(C)**. This molecule is famous for its beautiful "butterfly" structure, featuring one double-bonded oxygen and two peroxo (O-O) linkages.

The balanced chemical equation is:
$$Na_2CrO_4 + H_2SO_4 + 2H_2O_2 ightarrow CrO_5 + Na_2SO_4 + 3H_2O$$

Let's count the atoms in one molecule of $CrO_5$. We have 1 Chromium atom and 5 Oxygen atoms.
Total atoms in (C) = $1 + 5 = 6$.

### The Final Tally

We have successfully identified all three compounds and counted their atoms:
- Compound (A) is $CrO_2Cl_2$ with **5** atoms.
- Compound (B) is $Na_2CrO_4$ with **7** atoms.
- Compound (C) is $CrO_5$ with **6** atoms.

The question asks for the sum of the total number of atoms in one molecule each of (A), (B), and (C).
$$	ext{Sum} = 5 + 7 + 6 = 18$$

**The final answer is 18.** This problem beautifully connects qualitative analysis with basic stoichiometry, reminding us of the rich structural diversity of transition metal compounds!

Enter Numerical Value:

Visualized Solution

The Reaction Sequence

Reaction 1: Chromyl Chloride Test

Counting Atoms in $A$

Reaction 2: Formation of Chromate

Counting Atoms in $B$

Reaction 3: The Butterfly Molecule

Counting Atoms in $C$

Final Calculation

Structural Insights

The Sigma Insight: d-block Elements

The Chromyl Chloride Test

The Yellow Transformation

The Blue Butterfly

The Final Tally

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