Chemistry: Bonding, Moles, Solutions and Acids/Bases
Structure and bonding
Atoms consist of protons and neutrons in the nucleus with electrons in shells. Ionic bonds transfer electrons (metal plus nonmetal); covalent bonds share electrons (nonmetals). Periodic trends: reactivity of metals rises down a group, electronegativity rises across a period.
Moles and stoichiometry
The mole is 6.02 x 10^23 particles, and molar mass in g/mol converts grams to moles. Always balance the equation first, convert to moles, apply the mole ratio from the coefficients, then convert back to grams or litres. Identify the limiting reagent before predicting product yield; the excess reagent does not control the answer.
Solutions
Molarity = moles of solute / litres of solution. On dilution the moles of solute stay constant, so M1V1 = M2V2. Solubility often rises with temperature for solids.
Acids and bases
pH = -log[H+]. Strong acids ionize completely; weak acids only partly. Neutralization of acid plus base gives salt plus water, and buffers resist pH change. The pH scale is logarithmic, so a change of one unit means a tenfold change in hydrogen ion concentration.
Organic fundamentals
Recognize hydrocarbons (alkanes single bonds, alkenes double, alkynes triple) and functional groups: alcohol (-OH), aldehyde and ketone (C=O), carboxylic acid (-COOH), amine (-NH2). Isomers share a molecular formula but differ in structure.
Traps to expect
Confusing concentrated with strong; forgetting to balance before using coefficients; assuming dilution changes moles; treating pH 3 as slightly more acidic than pH 5.
Worked example
25 mL of 2.0 M HCl is diluted to 250 mL. M2 = M1V1/V2 = (2.0)(25)/250 = 0.20 M. Volume changed tenfold, so concentration fell tenfold.