In high school chemistry, especially for college entrance exams, mastering key formulas is essential for problem-solving. These formulas form the foundation of chemical calculations and are used throughout the entire curriculum.
The amount of substance, measured in moles, is a fundamental concept in chemistry. We calculate it using the formula n equals m over M, where n is the amount of substance, m is mass, and M is molar mass. For example, to find the amount of substance in 18 grams of water, we divide the mass by the molar mass of water, which is 18 grams per mole, resulting in 1 mole.
Molar concentration is another key concept, calculated using c equals n over V, where c is concentration, n is the amount of substance, and V is volume. For example, if we dissolve 0.5 moles of sodium chloride in 2 liters of water, the concentration would be 0.5 moles divided by 2 liters, which equals 0.25 moles per liter.
The ideal gas law, pV equals nRT, is crucial for understanding gas behavior. Here, p is pressure, V is volume, n is the amount of substance, R is the ideal gas constant (8.314 joules per mole kelvin), and T is temperature in kelvin. For example, at standard conditions (1 atmosphere pressure and 273 kelvin), one mole of an ideal gas occupies approximately 22.4 liters.
Chemical reaction rate measures how fast a reaction proceeds. It's defined as the change in concentration over time, v equals delta c over delta t. For example, if in a reaction from A to B, the concentration of A decreases by 0.5 moles per liter over 10 seconds, the reaction rate would be 0.05 moles per liter per second.
The equilibrium constant Kc expresses the relationship between concentrations of products and reactants at equilibrium. For a reaction aA plus bB in equilibrium with cC plus dD, Kc equals the concentration of C to the power of c times the concentration of D to the power of d, divided by the concentration of A to the power of a times the concentration of B to the power of b. Square brackets denote equilibrium concentrations.
Acid-base neutralization reactions are frequently tested in college entrance exams. The key reaction is hydrogen ion plus hydroxide ion forming water. At the equivalence point, the amount of substance of hydrogen ions equals the amount of hydroxide ions. For example, in 20 milliliters of 0.1 molar hydrochloric acid, the amount of hydrogen ions is 0.1 times 0.02, which equals 0.002 moles.
Electrochemical calculations involve electron transfer. We use the formula n of e minus equals I times t over F, where I is current in amperes, t is time in seconds, and F is Faraday's constant, approximately 96500 coulombs per mole. For example, with a current of 2 amperes for 10 minutes (600 seconds), the amount of transferred electrons is about 0.0124 moles.