NEET Chemistry - Chapter 7

Solutions

Fresh NEET solutions notes on concentration terms, ideal and non-ideal solutions, Raoult law, Henry law, and colligative properties.

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Concept Block

1. Concentration Terms: Mole Fraction, Molarity, Molality, and ppm

NEET tests the ability to convert between concentration terms and to choose the right one for a given context. The defining difference: molarity uses solution volume; molality uses solvent mass.

Term	Formula	NEET note
Mole fraction ( $x_A$ )	$n_A/(n_A+n_B)$	$x_A+x_B=1$ ; T-independent
Molarity (M)	$n_{solute}/V_{soln}$ (L)	T-dependent; most common in lab
Molality (m)	$n_{solute}/m_{solvent}$ (kg)	T-independent; use for colligative properties
Mass percent (w/w)	$(m_A/m_{soln})\times100$	T-independent
ppm	$(m_A/m_{soln})\times10^6$	Used for trace amounts (water quality)

Interconversion: For dilute aqueous solutions (density ≈ 1 g/mL): $m\approx M\times1000/(1000\times d - M\times M_B)$ . At dilution, $m\approx M$ for dilute aqueous.

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2. Raoult's Law, Ideal and Non-Ideal Solutions, and Azeotropes

Raoult's law: the vapour pressure of each component of an ideal solution equals its mole fraction times its pure-component vapour pressure.

p_A=x_A\cdot p_A^\circ,\quad p_B=x_B\cdot p_B^\circ

p_{total}=p_A+p_B=x_Ap_A^\circ+x_Bp_B^\circ

An ideal solution obeys Raoult's law at all compositions: $\Delta H_{mix}=0$ , $\Delta V_{mix}=0$ . Example: benzene + toluene.

Real solutions deviate:

Deviation	Cause	$p_{obs}$ vs ideal	Example
Positive	A–B interactions weaker than A–A, B–B	Higher	Ethanol + water, acetone + CS $_2$
Negative	A–B interactions stronger than A–A, B–B	Lower	Acetone + chloroform, HCl + water

Henry's Law: Solubility of gas in liquid is directly proportional to partial pressure of gas. $p = K_H \cdot x_{gas}$ . Higher $K_H$ → lower solubility. Temperature increase → more gas escapes → lower solubility.

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3. Colligative Properties: Four Key Expressions

Colligative properties depend only on the number of solute particles (moles), not on their identity. There are exactly four colligative properties NEET tests.

\frac{\Delta p}{p^\circ}=x_B\quad(\text{relative lowering of VP})

\Delta T_b=iK_bm\quad(K_b=\text{ebullioscopic constant})

\Delta T_f=iK_fm\quad(K_f=\text{cryoscopic constant})

\pi=iMRT\quad(\text{osmotic pressure; }M=\text{molarity})

Osmotic pressure is by far the most sensitive colligative property — it is used to measure the molecular mass of polymers and biomolecules.

In reverse osmosis, pressure greater than osmotic pressure is applied to the solution side to force solvent back through the membrane — used in water purification.

Worked example: For 0.1 m NaCl (fully dissociated,

i=2

) in water (

K_f=1.86

K kg mol

^{-1}

\Delta T_f = 2\times1.86\times0.1 = 0.372

K. Freezing point =

-0.372°

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4. van't Hoff Factor, Association, Dissociation, and Abnormal Molar Mass

The van't Hoff factor $i$ corrects colligative property formulas when solutes dissociate or associate in solution.

i=\frac{\text{observed colligative property}}{\text{calculated (normal) property}}=\frac{\text{actual particles in solution}}{\text{formula units dissolved}}

i=1+\alpha(n-1)\quad(\text{for dissociation, }n\text{ ions per formula unit, degree}\;\alpha)

i=1-\alpha(1-1/n)\quad(\text{for association into }n\text{-mer, degree}\;\alpha)

Situation	van't Hoff factor $i$	Molar mass (observed vs actual)
No change (non-electrolyte)	$i=1$	Equal
Dissociation (NaCl → 2 ions)	$i>1$	Observed < actual
Association (acetic acid in benzene)	$i<1$	Observed > actual

NEET trap: Acetic acid in benzene associates into dimers (

i\approx0.5

). Acetic acid in water partially dissociates (

i

slightly > 1). The solvent matters for association vs dissociation.

Concept Block

5. Ideal Dilute Solutions, Solubility Rules, and NEET Fast-Track

At infinite dilution, even non-ideal solutions obey Henry's law for the solute and Raoult's law for the solvent. This is called the ideal-dilute solution model.

Key facts for NEET rapid recall:

$K_f$ for water = 1.86 K kg mol $^{-1}$ ; $K_b$ for water = 0.512 K kg mol $^{-1}$
Osmotic pressure equation $\pi = iMRT$ is valid even for very dilute solutions
Colligative properties: Osmotic pressure > boiling point elevation > freezing point depression > vapour pressure lowering (in terms of sensitivity)
Isotonic solutions have equal osmotic pressures and show no net osmosis
Hypotonic solution placed in hypertonic environment → water leaves → cell shrinks (crenation)

NEET tip: For molar mass determination using osmotic pressure:

M_B = w_BRT/(\pi V)

. This is the most accurate method for high-MW polymers because even tiny concentrations give measurable osmotic pressure.

Practice Tests

5 Chapter Tests of 25 Questions Each

Each test is original, NEET-aligned, and answer-backed. Use them as sectional revision instead of a single long mock so your weak subtopics become easier to identify quickly.

Test 1: Concentration Basics

Mole fraction, mass percent, molarity, molality, and ppm.

Test 2: Vapour Pressure and Raoult Law

Ideal solutions, deviations, and partial-pressure ideas.

Test 3: Colligative Properties

Boiling-point elevation, freezing-point depression, and osmotic pressure.

Test 4: van't Hoff Factor

Association, dissociation, and particle-count interpretation.

Test 5: Mixed NEET Drill

Integrated numericals and concept questions across the full chapter.

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