What traumatic event does Zitkala-Sa describe at the school?

The forcible cutting of her long hair

Why was the cutting of her hair so humiliating to Zitkala-Sa?

Among her people, shingled hair was worn by cowards and mourners

Why are molality and mole fraction preferred over molarity in precise work?

They are independent of temperature, whereas molarity changes because volume varies with temperature.

State Henry's law and one application.

\(p=K_H x\); it explains why soda bottles are sealed under high CO\(_2\) pressure and why divers face decompression problems.

State Raoult's law for a solution of two volatile liquids.

For each component \(p_i=p_i^{\circ}x_i\), and the total pressure is the sum \(p=p_A^{\circ}x_A+p_B^{\circ}x_B\).

What causes a positive deviation from Raoult's law?

Weaker A–B interactions than A–A and B–B, raising vapour pressure above the ideal value (e.g. ethanol and acetone).

Define an ideal solution.

One that obeys Raoult's law over the entire range, with \(\Delta H_{mix}=0\) and \(\Delta V_{mix}=0\).

Why is the relative lowering of vapour pressure a colligative property?

It equals the mole fraction of the solute, which depends only on the number of solute particles present.

Write the expressions for elevation of boiling point and depression of freezing point.

\(\Delta T_b=K_b m\) and \(\Delta T_f=K_f m\), where \(K_b\) and \(K_f\) are molal constants.

Define osmotic pressure and give its equation.

The excess pressure needed to stop osmosis across a semipermeable membrane, \(\Pi=CRT\).

Why does NaCl give an abnormally high colligative property?

It dissociates into Na\(^+\) and Cl\(^-\), so the number of particles roughly doubles; \(i\approx 2\).

How is the van't Hoff factor defined?

It is the ratio of the observed colligative property to the value calculated assuming no association or dissociation.

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Chapter 1: Solutions — Class 12 Chemistry

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Chapter 1: Solutions

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Which of the following is a colligative property?

Summary

A solution is a homogeneous mixture of two or more components whose composition and properties are uniform throughout; the component in largest amount is the solvent. Concentration can be expressed as mass percentage, volume percentage, parts per million, mole fraction, molarity \(M\) (moles of solute per litre of solution) and molality \(m\) (moles per kg of solvent), of which mole fraction and molality are temperature independent. The solubility of a gas in a liquid is governed by Henry's law, \(p=K_H x\), the partial pressure of a gas over a solution being proportional to its mole fraction. For liquid-liquid solutions Raoult's law states that the partial vapour pressure of each volatile component is proportional to its mole fraction, \(p_A=p_A^{\circ}x_A\); ideal solutions obey it over the whole range while real solutions show positive or negative deviations. Properties that depend only on the number of solute particles and not their nature are colligative: relative lowering of vapour pressure, elevation of boiling point \(\Delta T_b=K_b m\), depression of freezing point \(\Delta T_f=K_f m\), and osmotic pressure \(\Pi=CRT\). These properties give molar masses of solutes; abnormal values arise when solutes associate or dissociate, accounted for by the van't Hoff factor \(i\).

Types of solutions and concentration unitsSolubility and Henry's lawVapour pressure and Raoult's lawIdeal and non-ideal solutionsColligative properties and molar massAbnormal molar mass and van't Hoff factor

Key terms

Mole fraction

Ratio of moles of one component to the total moles of all components; dimensionless and temperature independent.

Molality

Number of moles of solute per kilogram of solvent, \(m\); independent of temperature.

Henry's law

Partial pressure of a gas over a solution is proportional to its mole fraction, \(p=K_H x\).

Raoult's law

Partial vapour pressure of a volatile component equals its mole fraction times the pure-component vapour pressure, \(p_A=p_A^{\circ}x_A\).

Colligative property

A property that depends only on the number of solute particles, not their identity.

van't Hoff factor

Factor \(i\) that corrects colligative properties for dissociation or association of the solute.

Important questions

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Mole fraction

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Ratio of moles of one component to the total moles of all components; dimensionless and temperature independent.

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CBSE Class 12 — Solutions and Colligative Properties

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