Solutions

Definition

A solution is a homogeneous mixture of two or more substances. "Homogeneous" simply means that the mixture is uniform throughout; if you take a spoonful from the top and a spoonful from the bottom, they will be exactly the same.

Components of a Solution

• Solute: The substance that is being dissolved (usually present in smaller amounts).
• Solvent: The medium in which the solute is dissolved (usually present in larger amounts).

Types of Solutions

• Gaseous Solutions: Gas in Gas (e.g., Air).
• Liquid Solutions: Gas in Liquid (e.g., Aerated water), Liquid in Liquid (e.g., Ethanol in water), or Solid in Liquid (e.g., Salt in water).
• Solid Solutions: Solid in Solid (e.g., Alloys like Brass, which is Zinc in Copper).

Examples

• Saltwater: Salt (solute) dissolved in water (solvent).
• Brass: Zinc (solute) mixed into Copper (solvent).

Real-Life Use: Oceanography

The entire ocean is a massive, complex solution. Understanding the concentration of salts (salinity) is vital because it affects ocean currents, the density of water, and the survival of marine organisms.

Key Concentration Terms

• Mass Percentage (w/w %): (Mass of solute / Total mass of solution) × 100
• Volume Percentage (v/v %): (Volume of solute / Total volume of solution) × 100
• Mole Fraction (x): The ratio of moles of one component to the total moles of all components.
  x_A = n_A / (n_A + n_B)
• Molarity (M): Moles of solute per liter of solution. (Note: Changes with temperature)
• Molality (m): Moles of solute per kilogram of solvent. (Note: Temperature-independent)

Examples

• A 0.5 M Glucose solution: Means there are 0.5 moles of glucose in every 1 liter of that solution.
• A 10% w/w NaCl solution: Means there are 10g of salt in every 100g of the total solution.

Real-Life Use: Medical IV Drips

When a nurse administers an intravenous (IV) drip, the concentration must be exact. For instance, a "Normal Saline" solution is a specific molarity of salt. If the concentration is too high or too low, it can cause red blood cells to shrink or burst.

Definition

Solubility is the maximum amount of solute that can be dissolved in a specific amount of solvent at a given temperature.

Henry's Law

It states that at a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of the liquid.

Formula: P = K_H · x

(Where P = Partial pressure, K_H = Henry's Law constant, x = Mole fraction of gas in solution)

Key Points

• Higher pressure = More gas dissolved.
• Higher temperature = Lower gas solubility (this is why warm soda goes flat faster!).

Examples

• Soda Bottles: CO2 is dissolved under high pressure. When you open the cap, pressure drops, and gas escapes.
• Scuba Diving: As divers go deeper, pressure increases, and more nitrogen dissolves in their blood.

Raoult's Law Formula

For a solution of volatile liquids: P_total = P_A^0x_A + P_B^0x_B

Ideal vs. Non-Ideal Solutions

These are properties that depend only on the number of solute particles and not on the nature of the solute.

I. Relative Lowering of Vapour Pressure (RLVP)

Formula: (P^0 - P_s) / P^0 = x_solute

II. Elevation of Boiling Point (ΔT_b)

Formula: ΔT_b = K_b · m

III. Depression of Freezing Point (ΔT_f)

Formula: ΔT_f = K_f · m

IV. Osmotic Pressure (Π)

Formula: Π = CRT

Real-Life Use: Reverse Osmosis (RO)

By applying pressure greater than the osmotic pressure to salty water, we force water to move through a membrane, leaving contaminants behind.

When a solute dissociates into ions or associates, the number of particles changes.

The Van't Hoff Factor (i)

i = Observed Colligative Property / Calculated Colligative Property

• i > 1: Solute dissociated (e.g., NaCl $\rightarrow$ $i \approx 2$).
• i < 1: Solute associated (e.g., Acetic acid in benzene).

Updated Formulas

• $\Delta T_b = i \cdot K_b \cdot m$
• $\Pi = i \cdot CRT$

Problem 1: Molarity Calculation

Q: Calculate the molarity of a solution containing 5g of NaOH in 450 mL solution. (Atomic mass: Na=23, O=16, H=1)

Solution:

1. Moles of NaOH = 5 / 40 = 0.125 moles.
2. Volume = 0.45 L.
3. Molarity = 0.125 / 0.45 $\approx$ 0.278 M.

Problem 2: Boiling Point Elevation

Q: Calculate the boiling point of a solution prepared by dissolving 18g of glucose in 1kg of water. (Kb = 0.52 K kg/mol, Molar mass glucose = 180 g/mol).

Solution:

1. Moles of glucose = 18 / 180 = 0.1 moles.
2. Molality (m) = 0.1 / 1 = 0.1 m.
3. $\Delta T_b = 0.52 \times 0.1 = 0.052$ K.
4. Boiling Point = $100^\circ\text{C} + 0.052 = 100.052^\circ\text{C}$.

A pharmaceutical company is developing a new liquid medication. To ensure patient safety, the solution must be isotonic with human blood. If the solution is too concentrated (hypertonic), it will pull water out of red blood cells (crenation). If too dilute (hypotonic), cells will burst (hemolysis).

During a batch error, a technician used $CaCl_2$ instead of a non-electrolyte. The team noticed the osmotic pressure was nearly three times higher than expected. Using the Van't Hoff Factor (i), they realized $CaCl_2$ dissociates into three ions ($Ca^{2+} + 2Cl^-$), meaning $i = 3$. This mistake highlights why understanding dissociation is critical in life-saving chemistry.

1. Which of the following concentration terms is temperature-independent?
   (a) Molarity (b) Molality (c) Normality (d) Formality
   Answer: b
2. An isotonic solution has the same:
   (a) Molarity (b) Molality (c) Osmotic Pressure (d) Boiling Point
   Answer: c
3. Henry's law constant (K_H) increases with:
   (a) Increase in temperature (b) Decrease in temperature (c) Increase in pressure (d) None
   Answer: a
4. Which property is a colligative property?
   (a) Boiling point (b) Osmotic pressure (c) Surface tension (d) Viscosity
   Answer: b
5. The Van't Hoff factor for K_2SO_4 (assuming complete dissociation) is:
   (a) 1 (b) 2 (c) 3 (d) 4
   Answer: c
6. When a non-volatile solute is added to a solvent, the freezing point:
   (a) Increases (b) Decreases (c) Remains same (d) First increases then decreases
   Answer: b
7. An ideal solution obeys:
   (a) Raoult's Law (b) Henry's Law (c) Boyle's Law (d) Charles' Law
   Answer: a
8. The unit of Molality is:
   (a) mol/L (b) mol/kg (c) g/L (d) mol/mL
   Answer: b
9. Osmotic pressure is measured by:
   (a) Thermometer (b) Barometer (c) Osmometer (d) Manometer
   Answer: c
10. Which of the following shows negative deviation from Raoult's Law?
    (a) Ethanol + Acetone (b) Benzene + Toluene (c) Chloroform + Acetone (d) Water + Ethanol
    Answer: c

1. Q: Why does the vapor pressure of a solvent decrease when a non-volatile solute is added?
   A: Solute particles occupy some surface area, reducing the number of solvent molecules able to escape into the vapor phase.
2. Q: Differentiate between Molarity and Molality. Which one is preferred in labs?
   A: Molarity is moles/liter of solution (temp-dependent). Molality is moles/kg of solvent (temp-independent). Molality is preferred because it doesn't change with temperature.
3. Q: State Henry's Law and mention one application.
   A: P = K_H · x. Application: Carbonated drinks or scuba diving.
4. Q: What are Colligative Properties?
   A: Properties that depend only on the number of solute particles in a solution, not their chemical identity.
5. Q: Why is salt spread on icy roads in cold countries?
   A: To cause depression of the freezing point, making the ice melt at a temperature lower than 0°C.
6. Q: Define Osmotic Pressure. Write its formula.
   A: The pressure required to stop osmosis. Π = iCRT.
7. Q: What is the Van't Hoff factor for a substance that undergoes association?
   A: i < 1.
8. Q: Explain "Positive Deviation" from Raoult's Law with an example.
   A: When A-B interactions are weaker than A-A or B-B, vapor pressure is higher than expected. Example: Ethanol + Acetone.
9. Q: A solution of glucose in water is isotonic with a solution of urea. What can you say about their concentrations?
   A: Their molar concentrations are equal (since both are non-electrolytes, i=1).
10. Q: How can you determine the molar mass of a solute using freezing point depression?
    A: By measuring ΔT_f and using ΔT_f = K_f · m, where m contains the molar mass.

• Molarity vs. Molality: Molar-it-y is about Volume (think "V"); Molal-it-y is about Mass (think "M").
• Colligative Properties (The "Big Four"):
  • Relative Lowering (Vapour)
  • Elevation (Boiling)
  • Depression (Freezing)
  • Osmotic Pressure

Mnemonic: Remember the Colligative properties are "RED-O".