Osmotic Pressure and the van’t Hoff Factor

September 13, 2026

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Physical Chemistry · Solutions Osmotic Pressure and the van’t Hoff Factor The most sensitive of the colligative properties, and the only one practical for measuring the molar mass of very large molecules. BSc & MSc · Physical Chemistry · Concept The short answer: Osmotic pressure is the pressure that must be applied to stop solvent flowing through a semipermeable membrane into a solution. It is far larger than the other colligative effects at the same concentration, which is why it is the method of choice for polymers and proteins. The van’t Hoff factor corrects for dissociation or association of the…

Ionic Conductance, Kohlrausch’s Law and Transport Numbers

September 12, 2026

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Physical Chemistry · Electrochemistry Ionic Conductance, Kohlrausch’s Law and Transport Numbers Conductance measurements answer questions that look inaccessible — the degree of dissociation of a weak acid, and the solubility of a nearly insoluble salt. BSc & MSc · Physical Chemistry · Concept The short answer: Molar conductivity rises as a solution is diluted, but for different reasons in strong and weak electrolytes. Kohlrausch’s law of independent migration lets the limiting conductivity of a weak electrolyte be constructed from ionic values, which then gives its degree of dissociation and dissociation constant. The quantities Conductivity is conductance per unit dimension of…

Ideal and Non-Ideal Solutions: Raoult, Henry and Azeotropes

September 11, 2026

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Physical Chemistry · Solutions Ideal and Non-Ideal Solutions: Raoult, Henry and Azeotropes Deviations from ideality are not an inconvenience — they are the reason azeotropes exist, and the sign of the deviation predicts which kind forms. BSc & MSc · Physical Chemistry · Concept The short answer: An ideal solution obeys Raoult’s law across the whole composition range, which requires the interactions between unlike molecules to match those between like ones. Where unlike interactions are weaker, the solution shows positive deviation and can form a minimum boiling azeotrope; where stronger, negative deviation and a maximum boiling azeotrope. Raoult’s law and…

Collision Theory and Transition State Theory Compared

September 10, 2026

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Physical Chemistry · Kinetics Collision Theory and Transition State Theory Compared Two attempts to explain the same rate constant. One counts collisions; the other treats the transition state as a species with thermodynamic properties. BSc & MSc · Physical Chemistry · Concept The short answer: Collision theory predicts rate from collision frequency, an energy criterion and a steric factor, but the steric factor has to be fitted rather than calculated. Transition state theory instead assumes a quasi-equilibrium with an activated complex, giving the Eyring equation, whose parameters have direct thermodynamic meaning. Collision theory The reasoning is simple: molecules must collide…

Chromatography: The Principles Behind Every Variant

September 10, 2026

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Analytical Chemistry · Separation Chromatography: The Principles Behind Every Variant A dozen named techniques, one underlying idea — components separate because they spend different fractions of their time in a moving phase. BSc & MSc · Analytical Chemistry · Concept The short answer: Every form of chromatography has a stationary and a mobile phase. Components that interact more strongly with the stationary phase move more slowly. The retention factor quantifies this, resolution measures how well two peaks are separated, and the van Deemter equation explains what controls band broadening. The single underlying idea A mixture is carried by a mobile…

The Partition Function: Connecting Molecules to Thermodynamics

September 9, 2026

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Physical Chemistry · Statistical The Partition Function: Connecting Molecules to Thermodynamics One quantity that, once known, yields every thermodynamic property of the system. Understanding what it counts is more useful than memorising the formulas it generates. BSc & MSc · Physical Chemistry · Concept The short answer: The partition function sums a Boltzmann factor over every accessible state, so it measures how many states are effectively available at a given temperature. Energy, entropy and free energy all follow from it by differentiation, which is why it is the bridge between molecular properties and bulk thermodynamics. The definition and what it…

The Distribution Law and Why Extraction in Portions Works Better

September 9, 2026

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Physical Chemistry · Equilibria The Distribution Law and Why Extraction in Portions Works Better A short derivation that settles a practical question — is one large extraction better than several small ones, and by how much. BSc & MSc · Physical Chemistry · Concept The short answer: A solute distributes between two immiscible solvents in a fixed ratio at constant temperature. Working through the arithmetic shows that several small extractions remove far more solute than one extraction with the same total volume — which is why laboratory procedure always specifies portions. The law When a solute is shaken with two…

Photochemistry: Quantum Yield and the Jablonski Diagram

September 7, 2026

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Physical Chemistry · Photochemistry Photochemistry: Quantum Yield and the Jablonski Diagram Absorbing a photon puts a molecule somewhere unusual. The Jablonski diagram maps every route back down, and quantum yield counts which route was taken. BSc & MSc · Physical Chemistry · Concept The short answer: Quantum yield is the number of molecules undergoing a process per photon absorbed. The Jablonski diagram organises the competing routes back to the ground state — fluorescence, internal conversion, intersystem crossing and phosphorescence — and their relative rates decide what is observed. The two basic laws Grotthuss–Draper law: only light that is absorbed can…

Vibrational Spectroscopy: Force Constants and Anharmonicity

September 7, 2026

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Physical Chemistry · Spectroscopy Vibrational Spectroscopy: Force Constants and Anharmonicity The harmonic oscillator explains where the band is. Anharmonicity explains everything the harmonic model gets wrong, including why molecules can dissociate at all. BSc & MSc · Spectroscopy · Concept The short answer: A vibrating bond behaves approximately as a harmonic oscillator with evenly spaced levels and a selection rule of one quantum. The frequency depends on the force constant and the reduced mass, so a stiffer bond or lighter atoms absorb at higher wavenumber. Real bonds are anharmonic, which produces converging levels and permits overtones. The harmonic oscillator Model…

Rotational Spectroscopy: Measuring a Bond Length From a Spectrum

September 6, 2026

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Physical Chemistry · Spectroscopy Rotational Spectroscopy: Measuring a Bond Length From a Spectrum Line spacing gives the rotational constant, the rotational constant gives the moment of inertia, and that gives the bond length. Three steps, no ambiguity. BSc & MSc · Spectroscopy · Method The short answer: For a rigid diatomic rotor the energy levels are E = BJ(J+1), so successive transitions are separated by exactly 2B. Measure that spacing, extract B, convert to the moment of inertia and then to the bond length. The whole chain rests on the selection rule that J changes by one. The rigid rotor…

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