Some Basic Concepts of Chemistry

The One Thing

Quantitative chemistry connects the particles involved in matter and chemical reactions with measurable quantities such as mass, volume, amount of substance, and concentration. This connection depends on correct units, the mole concept, balanced equations, and stoichiometric relationships that obey conservation of atoms and mass.

Reactions, Processes and Experiments

Key Terms

• Matter: Anything that has mass and occupies space.
• Element: A pure substance made of only one kind of atom and represented by a chemical symbol.
• Compound: A pure substance formed when two or more elements combine chemically in a fixed ratio.
• Mixture: A physical combination of substances in variable proportions; its components retain their individual properties.
• Physical property: A property that can be observed or measured without changing a substance's chemical identity, such as density or melting point.
• Chemical property: A property describing how a substance undergoes chemical change, such as reactivity with oxygen.
• Law of conservation of mass: Mass is neither created nor destroyed during a chemical reaction, so the total mass of reactants equals the total mass of products.
• Law of definite proportions: A particular compound always contains the same elements in the same fixed ratio by mass.
• Law of multiple proportions: When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in simple whole-number ratios.
• Atomic mass: The relative mass of an atom compared with one-twelfth of the mass of a carbon-12 atom.
• Molecular mass: The sum of the atomic masses of all atoms present in one molecule.
• Formula mass: The sum of atomic masses represented in one formula unit of an ionic compound.
• Mole: The SI unit for amount of substance; one mole contains exactly 6.02214076 × 10^23 specified particles.
• Avogadro constant: The number of particles in one mole, equal to 6.02214076 × 10^23 mol^-1.
• Molar mass: The mass of one mole of a substance, expressed in g mol^-1; its numerical value equals the relative atomic, molecular, or formula mass.
• Molar volume: The volume occupied by one mole of a gas at a specified temperature and pressure; at 273.15 K and 1 bar it is approximately 22.7 L.
• Empirical formula: The simplest whole-number ratio of atoms of each element in a compound.
• Molecular formula: The actual number of atoms of each element in one molecule; it is a whole-number multiple of the empirical formula.
• Chemical equation: A symbolic representation of a chemical reaction using formulas and coefficients.
• Balanced chemical equation: An equation with equal numbers of atoms of every element on both sides, satisfying conservation of mass.
• Stoichiometry: The quantitative study of the amounts of reactants and products involved in a chemical reaction.
• Limiting reagent: The reactant consumed first, which limits the maximum amount of product formed.
• Excess reagent: A reactant present in more than the amount required by the balanced equation; some remains after the reaction.
• Theoretical yield: The maximum amount of product predicted from stoichiometric calculations.
• Actual yield: The amount of product obtained experimentally.
• Percentage yield: The efficiency of a reaction, calculated as actual yield divided by theoretical yield multiplied by 100.
• Molarity: The number of moles of solute present in one litre of solution, expressed in mol L^-1.
• Molality: The number of moles of solute present in one kilogram of solvent, expressed in mol kg^-1.
• Mass percentage: The mass of a component divided by the total mass of the mixture or solution, multiplied by 100.
• Parts per million: A concentration unit representing one part of solute per one million parts of solution or mixture, commonly used for very dilute systems.
• Significant figures: Digits in a measured quantity that include all certain digits and the first uncertain digit.
• Dimensional analysis: A calculation method that uses units to convert quantities and check whether an equation is consistent.

Easily Confused

• Element and compound: An element contains one kind of atom, whereas a compound contains two or more elements chemically combined in a fixed ratio.
• Compound and mixture: A compound has fixed composition and new chemical properties, whereas a mixture has variable composition and its components retain their individual properties.
• Physical property and chemical property: A physical property is measured without changing chemical identity, whereas a chemical property describes how a substance undergoes chemical change.
• Atomic mass, molecular mass and formula mass: Atomic mass refers to one atom, molecular mass is the sum of atomic masses in one molecule, and formula mass is the sum represented in one ionic formula unit.
• Empirical formula and molecular formula: The empirical formula gives the simplest whole-number ratio, whereas the molecular formula gives the actual number of atoms in one molecule.
• Molarity and molality: Molarity uses litres of solution, whereas molality uses kilograms of solvent.
• Limiting reagent and excess reagent: The limiting reagent is consumed first and restricts product formation, whereas the excess reagent remains partly unreacted.
• Theoretical yield and actual yield: Theoretical yield is the calculated maximum, whereas actual yield is the amount obtained experimentally.
• Mass percentage and parts per million: Mass percentage expresses composition per hundred, whereas parts per million expresses very dilute composition per million.
• Multiplication or division and addition or subtraction of measurements: Multiplication and division are limited by significant figures, whereas addition and subtraction are limited by decimal places.

What Gets Asked

• Define and distinguish matter, elements, compounds, mixtures, physical properties, and chemical properties; marks are lost by treating mixtures as substances with fixed composition.
• State or apply the laws of conservation of mass, definite proportions, and multiple proportions; marks are lost by failing to identify fixed or simple whole-number ratios.
• Calculate moles, mass, particles, density, or gas volume using the stated equations; marks are lost by omitting units or using molar volume without specified temperature and pressure conditions.
• Determine an empirical or molecular formula; marks are lost by failing to convert mass or percentage data into moles, divide by the smallest value, and convert to the smallest whole-number ratio.
• Balance a chemical equation and use its coefficients as mole ratios; marks are lost by treating coefficients as mass ratios or performing stoichiometric calculations before balancing.
• Identify the limiting and excess reagents and calculate theoretical yield, actual yield, or percentage yield; marks are lost by selecting the reactant with the smaller mass rather than comparing available moles with balanced-equation coefficients.
• Calculate molarity, molality, mole fraction, mass percentage, or parts per million; marks are lost by confusing volume of solution with mass of solvent.
• Apply significant-figure rules and dimensional analysis; marks are lost by using significant figures for addition and subtraction where decimal-place rules are required, or by producing an inconsistent final unit.