Chapter 14, Problem 5STP

Interpretation Introduction

Interpretation:

The solute showed the greatest effect on the vapour pressure of its respective solution have to be identified.

Concept introduction:

Colligative properties are those properties of solutions, which entirely depend upon the number of solute particles (molecules or ions) dissolved in a known volume of solvent, but not at all on the nature (i.e., chemical composition or constitution) of those solute particles. Here the solute is invariably taken as non-volatile. There are four colligative properties, which are lowering of vapor pressure, osmotic pressure, elevation of boiling point and depression of freezing point.

Lowering of vapor pressure: When the solvent contains solute, a mix of solute and solvent particles occupies the surface area. When there is fewer solvent particles at the surface, then fewer particles will enter the gaseous state, and that results in lowering of vapor pressure. The higher the number of solute particles in a solvent, the lower will be the resulting vapor pressure.

Answer to Problem 5STP

Answer is option C. MgCl₂ will produce the most number of particles in solution: 1 mol Mg²+ ion and 2 mol Cl-ion.

Explanation of Solution

When there is fewer solvent particles at the surface, then fewer particles will enter the gaseous state, and that results in lowering of vapor pressure. Thus, vapor pressure lowering is because of the number of solute particles in solution. Among the given four solutes, three solutes are KBr, MgCl₂ and CaSO₄ are electrolytes and they gave ions in the aqueous solution whereas C₆H₁₂O₆ is a non-electrolyte. Solute containing the greater number of solute particles in a solvent will show greatest effectin the lowering vapor pressure. MgCl₂ produces three ions in the solution whereas both KBr and CaSO₄ give two ions in the solution. As colligative properties are entirely depend upon the number of solute particles (molecules or ions) dissolved in a known volume of solvent. So, MgCl₂shows the greatest effect.

Conclusion

Reasons for incorrect options:

As, it is already told that MgCl₂ produces maximum number of ions in the solution than both KBr and CaSO₄ whereasC₆H₁₂O₆ is a non-electrolyte. As colligative properties are entirely depend upon the number of solute particles (molecules or ions) dissolved in a known volume of solvent. So, MgCl₂shows the greatest effect.

Answer is C. The product is MgCl₂. That means other options that is,

A. KBr

B. C₆H₁₂O₆

D. CaSO₄are found to be incorrect.