Problem 152 - A Complete Resource Book in Chemistry for JEE Main

Question

The coagulation of \(100 \mathrm{ml}\) of colloidal solution of gold is completely prevented by addition of \(0.25 \mathrm{~g}\) of a substance " \(\mathrm{X}\) " to it before addition of \(10 \mathrm{ml}\) of \(10 \% \mathrm{NaCl}\) solution. The gold number of "X" is (a) 25 (b) 250 (c) \(2.5\) (d) \(0.25\)

Step-by-Step Solution

Answer

The gold number of X is 25.

Step 1: Understand the Gold Number

The gold number is defined as the minimum mass (in milligrams) of a protective colloid required to prevent the coagulation of 10 ml of a gold sol when 1 ml of a 10% NaCl solution is added to it. It serves as a measure of the protective power of a colloid. Lower the gold number, better is the protection.

Step 2: Calculate the Gold Number

In this problem, 0.25 g of substance X is able to prevent the coagulation of 100 ml of gold sol when 10 ml of 10% NaCl solution is added. First, convert 0.25 g to milligrams: \[ 0.25 ext{ g} = 250 ext{ mg} \].Since this is for 100 ml, we need to adjust it for 10 ml.The amount needed for 10 ml of gold sol (since the gold number definition is for 10 ml) would be:\[ \frac{250 ext{ mg}}{10} = 25 ext{ mg}\].Therefore, the gold number of X is 25.

Key Concepts

Coagulation

Coagulation is an essential concept in chemistry, especially when dealing with colloidal solutions. Coagulation refers to the process where particles in a colloidal solution aggregate and form larger particles. These aggregated particles can settle out of the solution, leading to the precipitation of the colloid.
This process can be induced by several methods, such as adding electrolytes. When an electrolyte like NaCl is introduced, it can neutralize the charged particles in the solution, causing them to clump together.

• This clumping effect leads to the larger particles settling out.
• It disrupts the stability of the colloid, which is key to its unique properties.

Understanding coagulation helps us manipulate and control colloidal properties for various applications.

Protective Colloid

A protective colloid is a substance that can prevent or significantly delay the coagulation of a colloidal solution. When added to a colloidal solution, it stabilizes the colloid by forming a protective layer around the suspended particles.
This protective layer acts as a barrier, preventing aggregation from occurring even in the presence of electrolytes.

• The effectiveness of a protective colloid is measured by its protective power.
• Colloidal substances with lower gold numbers are generally better protective colloids.

Protective colloids are crucial in industries where maintaining colloidal stability is vital, such as in food production and pharmaceuticals.

Colloidal Solution

A colloidal solution, or colloid, is a mixture where very small particles of one substance are evenly dispersed throughout another substance. These particles are larger than molecules but still small enough to remain suspended throughout the continuous phase.
Unlike true solutions, the particles in a colloid do not settle out quickly and cannot be separated easily by filtration.

• Colloids can appear in different forms like sol, gel, and emulsion.
• The dispersed phase and dispersion medium have different phases, such as solid, liquid, or gas.

Colloidal solutions have distinct properties like the Tyndall effect, where light is scattered by the colloidal particles, making the beam visible.

NaCl Solution Effect

Adding a NaCl solution to a colloidal mixture can have significant effects due to the electrolyte properties of NaCl. When sodium chloride, a common salt, is added to a colloidal solution, it can lead to coagulation. This is because NaCl dissociates into sodium ( Na^+ ) and chloride ( Cl^- ) ions, which interact with the charged particles in the colloid.
These ions neutralize the charges, thus reducing the repulsive forces between particles.

• This causes the particles to come closer and aggregate.
• The formation of larger aggregates eventually leads to their precipitation out of the solution.

It's important to control the amount of NaCl added to maintain colloidal stability, which is crucial for applications requiring precise control over solution characteristics.

Protective Power of Colloids

The protective power of colloids refers to their ability to prevent the coagulation of a colloidal solution. When a protective colloid is added, it ostensibly attaches to the particles in the sol, forming a barrier that shields them against the effects of added electrolytes.
The gold number is a parameter used to quantify this protective power. It is defined as the minimum amount of a protective colloid required to prevent the coagulation of a colloidal solution in the presence of added electrolytes.

• The lower the gold number, the more effective the colloid is at providing protection.
• This concept is vital for applications where colloidal stability is essential, such as in paint formulations and food products.

Understanding the protective power helps in designing formulations that remain stable under various conditions.