Chemistry - General, Organic and Introductory Biochemistry
The chemistry section covers stoichiometry, solutions, equilibrium, acid–base chemistry, organic functional groups and mechanisms, and introductory biochemistry (amino acids, carbohydrates, lipids, cofactors). Items are short calculations or structure-based discriminations, so precision with units and stereochemistry decides the mark.
Core rules
- Moles and stoichiometry. Convert every quantity to moles; the limiting reagent is the one giving the smallest product yield. Molarity = mol/L and, on dilution, M₁V₁ = M₂V₂ - volume changes, moles do not.
- Acid–base. pH = −log[H⁺]; at 25 °C pKw = 14. Buffers obey pH = pKa + log([base]/[acid]); capacity is maximal at pH = pKa (1:1 ratio) and falls as the ratio moves away from 1.
- Equilibrium. K depends only on temperature; a catalyst changes rate, never the equilibrium position.
- Organic. Naming priority: carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine. Assign R/S by Cahn–Ingold–Prelog after pointing the lowest-priority group away. SN2 is concerted with inversion; SN1 passes through a carbocation and racemises.
- Biochemistry. Amino acids are zwitterionic near neutral pH; peptide bonds are planar and rigid; α-helices and β-sheets are held mainly by backbone hydrogen bonds.
Worked example
Equal volumes of 0.10 M acetic acid (pKa 4.76) and 0.05 M sodium acetate are mixed. Because both are diluted by the same factor, the ratio stays 2:1 acid:base, so pH = 4.76 + log(0.05/0.10) = 4.76 − 0.30 = 4.46. Diluting the mixture tenfold with water leaves the pH unchanged but drops the buffer capacity sharply.
Traps
Inserting moles instead of concentrations into the buffer ratio; assuming dilution shifts buffer pH; confusing Ka with Kb; calling a carbon chiral when two substituents are identical.