In Carius method for estimation of halogens, 180 mg of an organic compound produced 143.5 mg of AgCl. The percentage composition of chlorine in the compound is ______ %. [Given : molar mass in g mol –1 of Ag : 108, Cl = 35.5]

Key Concepts

1. Carius Method (Gravimetric Analysis)

   • Involves heating an organic compound with fuming $HNO_3$ and $AgNO_3$ in a sealed hard-glass tube.
   • Any halogen (Cl, Br, I) present forms an insoluble silver halide (AgCl, AgBr, AgI).
   • The silver halide is filtered, washed, dried, and weighed.

2. Stoichiometry of AgCl Formation

   • Reaction: $\text{R–Cl} + Ag^+ \;\longrightarrow\; AgCl \downarrow + \text{R}^+$
   • The molar ratio is 1 mol Cl : 1 mol AgCl.

3. Mass–Mole–Mass Chain

   1. convert mass of AgCl → moles of AgCl using molar mass $M_{AgCl} = M_{Ag} + M_{Cl} = 108 + 35.5 = 143.5\,\text{g mol}^{-1}$
   2. moles of AgCl = moles of Cl (1 : 1)
   3. convert moles of Cl → mass of Cl using $M_{Cl} = 35.5\,\text{g mol}^{-1}$
   4. percentage $\frac{\text{mass of Cl}}{\text{mass of compound}} \times 100$

Logical Flow for the Student

• Why weigh AgCl? Because it’s easy to isolate and every particle tells us exactly one chlorine atom is present.
• Why convert to moles? Stoichiometric relationships are always in moles.
• Why divide by original sample mass? Percentage means ‘parts of chlorine per 100 parts of sample’.

What’s happening here?

Imagine you have a cookie (the organic compound) that secretly contains some salt (chlorine). You melt the cookie in a special oven called Carius tube so that every chlorine atom pairs up with a silver atom to form shiny silver-chloride crystals (AgCl)—just like salt that sparkles.

We weigh:

• The original cookie: 180 mg
• The sparkly salt (AgCl) we got: 143.5 mg

Because every speck of AgCl has exactly one chlorine atom inside, by figuring out how much chlorine is inside those 143.5 mg, we can tell how much chlorine was hiding in the cookie. Finally we compare that chlorine weight to the original 180 mg cookie weight to find the percentage of chlorine inside.

That’s all the question wants: “What percent of my cookie is chlorine?”

Step-by-Step Solution

1. Data

   • Mass of compound, $m_{sample} = 180\,\text{mg} = 0.180\,\text{g}$
   • Mass of AgCl, $m_{AgCl} = 143.5\,\text{mg} = 0.1435\,\text{g}$
   • $M_{AgCl} = 108 + 35.5 = 143.5\,\text{g mol}^{-1}$
   • $M_{Cl} = 35.5\,\text{g mol}^{-1}$

2. Moles of AgCl formed

3. Moles of chlorine present

Since $AgCl$ contains 1 Cl per molecule,

4. Mass of chlorine

5. Percentage of chlorine in the compound

Final Answer: $\boxed{19.7\%}$