Uploaded August 2026 | Updated September 2026, 1 week ago

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Molarity, Molality, and Normality
Use molarity when dealing with solution chemistry where volume is easy to measure.

Use molality when temperature varies or when studying boiling/freezing point changes.

Use normality when the reaction involves charge or proton transfer, and equivalents matter.
Molarity (M)
Molarity is the concentration of a solution expressed as moles of solute per liter of solution.
Formula:
𝑀="moles of solute" /"liters of solution"
Key Features:
Depends on volume, which changes with temperature.
Commonly used in aqueous solutions, titrations, and equilibrium calculations.

Molality (m)
Molality (m): is the concentration expressed as moles of solute per kilogram of solvent.
Formula:
𝑚="moles of solute" /"kg of solvent"
Key Features:
Depends on mass, not volume → temperature‑independent.
Used in colligative properties (boiling point elevation, freezing point depression).

Normality (N)
Normality (N): measures concentration in equivalents per liter of solution.
Formula:
𝑁=𝑀×("number of equivalents" )
What Counts as an “Equivalent”? Depends on the reaction type:
Acid–base: H⁺ or OH⁻ donated/accepted
Redox: electrons transferred
Precipitation: charge equivalents
Example: 1 M H₂SO₄ → 2 equivalents of H⁺ → 2 N

Core Differences
Molarity: moles of solute per liter of solution
Temperature‑dependent (volume changes)
Most common in equilibrium, kinetics, and titrations

Molality: moles of solute per kilogram of solvent
Temperature‑independent (mass does not change)
Used for colligative properties

Normality: equivalents per liter of solution
Reaction‑specific (acid–base, redox, precipitation)
Useful when stoichiometry involves equivalents rather than moles
Preparing Solutions

Preparing solutions: Always specify if molarity is for the hydrate/salt form (ex. MgCl₂·6H₂O)

Dilutions: C₁V₁ = C₂V₂ (valid for molarity in dilute aqueous solutions).

Understanding the C₁V₁ = C₂V₂ Equation
Dilution is about making a solution less concentrated by adding solvent (usually water). The equation:
𝐶_1 𝑉_1=𝐶_2 𝑉_2
C₁ = initial concentration
V₁ = volume you need to take from the stock
C₂ = final concentration
V₂ = final total volume after dilution


Why it works: The number of moles stays the same before and after dilution
*only the volume changes.

Step‑by‑Step: How to Prepare a Diluted Solution
Identify C₁, V₂, and C₂.
Solve for V₁ using
𝑉_1=(𝐶_2 𝑉_2)/𝐶_1
Measure V₁ of the stock solution.
Transfer to a volumetric flask.
Add solvent until you reach V₂.

Definition of pH
pH=−log⁡10[H+]
Each 1‑unit change in pH = 10‑fold change in [H+]
Temperature Dependence
Kw increases with temperature

Neutral pH drops as temperature rises

At 37 °C, neutral pH ≈ 6.8

Important for interpreting physiological pH (blood ≈ 7.4 is still alkaline relative to neutrality)
Strong Acids and Bases
Strong acids and strong bases dissociate completely in water
Examples:
HCl → H⁺ + Cl⁻
NaOH → Na⁺ + OH⁻
In dilute solutions: [H⁺] ≈ initial concentration

Osmosis in Cells
Water moves from higher Ψ → lower Ψ across membranes.
If a cell is placed in a solution with lower Ψ → water leaves → plasmolysis
If placed in higher Ψ → water enters → turgor pressure increases

Pressure Potential (Ψp): The Push of Water
Pressure potential is the physical pressure exerted on water.
In turgid plant cells, Ψp is positive (cell wall pushes back).
In xylem, Ψp can be negative due to tension from transpiration.
Pressure can raise water potential, helping water move upward.

Movement Through Plant Tissues
Water potential gradients drive:
Root uptake
Xylem transport
Leaf transpiration

Water moves from soil (highest Ψ) → roots → stem → leaves → air (lowest Ψ).

Turgor Pressure and Cell Function
Turgor pressure maintains:
Leaf rigidity
Growth
Stomatal opening
Low Ψ in the environment → loss of turgor → wilting.

Water Potential and the Cohesion–Tension Mechanism
Transpiration creates negative pressure potential in leaves:
Water evaporates
Pulls water upward through xylem
Cohesion keeps the column intact
Adhesion helps water stick to xylem walls
This entire process is driven by Ψ gradients.

Gravity is overcome by pressure potential + solute potential + transpiration tension.
Typical Ψ values in a transpiring plant
Soil: Ψ ≈ –0.1 MPa
Root xylem: Ψ ≈ –0.3 MPa
Stem xylem: Ψ ≈ –0.6 MPa
Leaf mesophyll: Ψ ≈ –1.5 MPa
Air (dry): Ψ ≈ –100 MPa or lower
This enormous gradient explains how water is pulled upward against gravity.


Link to Lecture Slides: drive.google.com/file/d/1mpj28HsfUezYR-QpF3iSDAS9juxqPJPj/view?usp=drive_link

Due to the description character limit the full work cited for "Preparing Solutions Understanding pH and Water Potential" can be viewed at... docs.google.com/document/d/1ARYz5HO1Do9joQZ0suC1PpF0xjbx3FSS/edit?usp=drive_link&ouid=104237452697237972847&rtpof=true&sd=true