Chemistry: Structure, Reactions, and Calculations
Core concepts to master
Atomic structure underpins everything: protons define the element, neutrons affect mass, and electrons govern bonding. Know the periodic trends, including atomic radius (decreases across a period), electronegativity (increases across a period), and ionization energy. Distinguish ionic from covalent bonding and predict polarity from electronegativity differences. Master stoichiometry: balancing equations, the mole concept, molar mass, limiting reagents, and percentage yield. Solutions require molarity, dilution, and pH. Acid-base chemistry involves the Bronsted-Lowry definitions, strong versus weak acids, and neutralization. Oxidation-reduction questions test oxidation states and electron transfer.
Traps to watch
Unbalanced equations are the most common source of stoichiometry errors, so balance before calculating. Do not confuse molarity (moles per liter of solution) with molality (moles per kilogram of solvent). Remember that a strong acid fully dissociates while a weak acid only partly does, so pH calculations differ. In limiting-reagent problems, identify which reactant runs out first rather than assuming the larger quantity limits. Watch units: convert grams to moles and milliliters to liters.
Worked example
How many grams of water form when 4 g of hydrogen reacts with excess oxygen? The balanced equation is 2H2 + O2 to 2H2O. Moles of H2 equal 4 g divided by 2 g/mol, giving 2 mol. The ratio of H2 to H2O is 1:1, so 2 mol of water forms. Mass equals 2 mol times 18 g/mol, which is 36 g. Rule of thumb: convert to moles, use the balanced ratio, then convert back to the requested unit.