Solutions and Concentration: Chemistry

Solutions and Types of Solutions

• Solutions are homogenous mixtures where components are uniformly distributed
• Solutions consist of a solute and a solvent
• Solutes are the substance being dissolved and present in smaller quantity
• Solvents are the substance doing the dissolving and present in larger quantity
• Types of solutions include gaseous, liquid, and solid solutions, based on the physical state of the solvent

Concentration Terms

• Concentration expresses the amount of solute present in a given quantity of solution
• Molarity (M) is defined as the number of moles of solute per liter of solution (mol/L)
• Molality (m) is defined as the number of moles of solute per kilogram of solvent (mol/kg)
• Normality (N) is defined as the number of gram equivalents of solute per liter of solution
• Mole fraction (x) is defined as the ratio of the number of moles of a component to the total number of moles in the solution

Strength and Molarity

• Strength refers to the mass of solute in grams present in one liter of solution, expressed as g/L
• Molarity (M) is the number of moles of solute dissolved in one liter of solution, calculated as: M = (Number of moles of solute) / (Volume of solution in liters)
• Molarity is temperature-dependent because the volume of a solution changes with temperature

Molality

• Molality (m) is defined as the number of moles of solute per kilogram of solvent
• The formula for molality is: m = (moles of solute) / (mass of solvent in kg)
• Unlike molarity, molality is not temperature-dependent since it relies on mass, which remains constant with temperature changes

Normality

• Normality (N) is the number of gram equivalent weights of solute present in one liter of solution
• Equivalent weight is the molar mass divided by the n-factor which is the number of reactive units per molecule
• For acids, the n-factor is the number of replaceable H+ ions; for bases, it's the number of replaceable OH- ions

NCERT Points

• Solutions are homogeneous mixtures with uniform composition throughout
• The components of a solution are the solute and the solvent
• The concentration of a solution can be expressed in terms of molarity, molality, normality, and mole fraction

Solubility

• Solubility is the maximum amount of a solute that can dissolve in a given amount of solvent at a specific temperature
• Factors affecting solubility include temperature, pressure, and the nature of the solute and solvent
• "Like dissolves like" principle: polar solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents

Solubility of Solids in Liquids

• Temperature affects the solubility of solids in liquids
• An increase in temperature usually increases the solubility of solids in liquids (endothermic)
• Pressure has no significant effect on the solubility of solids in liquids because solids and liquids are incompressible

Effect of Pressure

• Pressure significantly affects the solubility of gases in liquids but not solids or liquids
• The solubility of a gas in a liquid increases with an increase in pressure

Solubility of Gas in Liquids

• Henry's Law states that the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the solution
• The equation for Henry's Law is: P = KH x, where P is the partial pressure of the gas, KH is Henry's Law constant, and x is the mole fraction of the gas in the solution
• Factors affecting the solubility of gases in liquids include temperature (solubility decreases with increasing temperature) and the nature of the gas and solvent

Henry’s Law: Limitations and Applications

• Limitations of Henry's Law include: the law is applicable at low pressures and high temperatures, the gas should not react with the solvent, and the gas should not associate or dissociate in the solution
• Applications of Henry's Law is in the production of carbonated beverages (CO2 solubility increases under high pressure), in deep-sea diving (to understand the bends), and in respiration

Vapour Pressure

• Vapour pressure is the pressure exerted by the vapor of a liquid when the liquid is in equilibrium with its vapor
• Factors affecting vapor pressure include temperature (vapor pressure increases with increasing temperature) and the nature of the liquid

Liquid Solutions

• Raoult's Law states that for a solution of volatile liquids, the partial vapor pressure of each component is directly proportional to its mole fraction in the solution
• For an ideal solution: ΔH mixing = 0, ΔV mixing = 0, and it obeys Raoult's Law

Types of Solutions

• Ideal solutions obey Raoult's Law: Ptotal = P0A xA+ P0B xB
• Non-ideal solutions do not obey Raoult's Law and show positive or negative deviations
• Positive deviation occurs when the vapor pressure is higher than predicted
• Negative deviation occurs when the vapor pressure is lower than predicted.

Azeotropes

• Azeotropes are binary mixtures that have the same composition in the liquid and vapor phases and boil at a constant temperature
• Minimum boiling azeotropes show a positive deviation from Raoult's Law
• Maximum boiling azeotropes show a negative deviation from Raoult's Law

Raoult’s Law as a Special Case of Henry’s Law

• Raoult's Law is a special case of Henry's Law where the constant KH is equal to the vapor pressure of the pure solvent

Colligative Properties

• Colligative properties are properties of solutions that depend on the number of solute particles but not on the nature of the solute
• Four main colligative properties: relative lowering of vapor pressure, elevation of boiling point, depression of freezing point, and osmotic pressure

Relative Lowering of Vapour Pressure

• Relative lowering of vapor pressure is proportional to the mole fraction of the solute in the solution
• ΔP/P0 = xsolute, where ΔP is the lowering of vapor pressure, P0 is the vapor pressure of the pure solvent, and xsolute is the mole fraction of the solute

Elevation of Boiling Point

• Elevation of boiling point (ΔTb) is directly proportional to the molality (m) of the solution
• ΔTb= Kb m, where Kb is the ebullioscopic constant

Depression in Freezing Point

• Depression in freezing point (ΔTf) is directly proportional to the molality (m) of the solution
• ΔTf= Kf m, where Kf is the cryoscopic constant

Osmotic Pressure

• Osmotic pressure (π) is the pressure required to prevent the flow of solvent into the solution through a semipermeable membrane
• π = CRT, where C is the molar concentration, R is the gas constant, and T is the absolute temperature

Example of Osmosis

• Example of osmosis is the swelling of a raisin when placed in water due to the movement of water molecules

Van't Hoff Factor

• The van't Hoff factor (i) is the ratio of the actual number of particles in solution after dissociation or association to the number of particles initially dissolved in solution
• i = (Number of particles after dissociation or association) / (Number of particles initially dissolved)
• For dissociation, i > 1; for association, i < 1; for non-electrolytes, i = 1

Abnormal Molar Mass

• Abnormal molar mass occurs when the experimental molar mass differs from the theoretical molar mass due to dissociation or association of solute particles
• Molar mass is inversely proportional to colligative properties