Summary
Highlights
The mole is a fundamental unit in chemistry for measuring the amount of substance. One mole of any substance contains Avogadro's number (6.02 × 10^23) of particles (atoms or molecules). The number of particles can be calculated by multiplying Avogadro's number by the number of moles.
For solids, the number of moles is calculated by dividing the mass (in grams) by the molecular mass (Mr) for compounds or atomic mass (Ar) for elements. This formula can be rearranged to find mass or Mr/Ar. It's crucial to pay attention to significant figures and recall that if in doubt, one should work out the moles.
For solutions, the number of moles is found by multiplying concentration (in moles per dm³) by volume. The volume must be in dm³, so conversions from cm³ (by dividing by 1000 or multiplying by 10^-3) are often necessary. This formula is vital for calculations involving solutions and can be rearranged as needed.
The ideal gas equation (PV=nRT) is used to relate pressure (P), volume (V), number of moles (n), gas constant (R), and temperature (T) for gases. Units are critical: pressure in Pascals, volume in meters cubed, and temperature in Kelvin (add 273 to Celsius). The gas constant (R = 8.31 J mol⁻¹ K⁻¹) is usually provided.
Converting between meters (m), decimeters (dm), and centimeters (cm) is essential, especially for volume. To convert m³ to dm³ or dm³ to cm³, multiply by 1000. To convert m³ to cm³, multiply by 1,000,000. Going the opposite way involves division. This is crucial for accurate calculations in all areas of amount of substance.
Ionic equations show only the ions that participate in a chemical reaction. Spectator ions, which remain unchanged on both sides of the reaction, are removed to simplify the equation. This is particularly useful for reactions in solution involving acids, bases, and salts. The charges must balance on both sides.
Balanced chemical equations can be used to calculate the theoretical mass of a product that can be formed from a given mass of reactant. This involves using the Mr or Ar values and mole ratios from the balanced equation to establish a proportional relationship between reactants and products. Working out moles is often the first step.
Titrations are experimental procedures used to determine the concentration of an unknown acid or alkali. This involves using a burette and a conical flask, adding an indicator to detect the endpoint, and repeating measurements for concordant results (within 0.1 cm³). Calculations use the moles = concentration × volume formula, alongside mole ratios from balanced equations.
The empirical formula represents the simplest whole-number ratio of elements in a compound. It can be determined from percentage compositions or masses of elements, by finding the moles of each element and dividing by the smallest mole value. The molecular formula is a multiple of the empirical formula and requires the Mr of the compound.
Percentage yield is calculated as (actual yield / theoretical yield) × 100 and indicates the efficiency of a reaction in terms of product obtained. Atom economy measures how many atoms from the reactants are incorporated into the desired product, calculated as (molecular mass of desired product / sum of molecular masses of all reactants) × 100. High atom economy is desirable for sustainability, reduced waste, and cost efficiency.