Some basic concepts of Chemistry

• Chemistry is a branch of science that deals with the study of matter, its composition, structure, and reactions (transformations)
• Matter is anything that has mass and occupies space
• Matter exists in three physical states: solid, liquid, and gas
• Solids:

  • Particles are closely held together in an orderly fashion
  • Have a definite shape and a definite volume
• Liquids:

  • Particles are closely together but are able to move around
  • Have a definite volume but not a definite shape
• Gas:

  • Made up of atoms or molecules that are far apart and move independently in all directions with random motion
  • Do not have a definite volume or a definite shape
• Pure substances are made up of only one type of particles with a fixed ratio

  • Pure substances are either elements or compounds and always have a homogeneous composition
• Elements consist of only one type of particles, either atoms or molecules

  • Examples: Sodium, copper, silver, Hydrogen, oxygen
• Compounds are formed when two or more atoms of different elements are combined

  • Example: Water = Hydrogen + Oxygen, Ammonia = Hydrogen + Nitrogen
• Mixtures always contain two or more substances in any ratio and may be either homogeneous or heterogeneous
• Homogeneous Mixtures:

  • Components are completely mixed and have a uniform composition
  • Components can be separated by chemical methods (e.g. evaporation)
  • Example: Salt solution, Sugar solution
• Heterogeneous Mixtures:

  • Components are not completely mixed and do not have a uniform composition
  • Components can be separated by physical methods
  • Example: Salt + Sugar, Sand + mud
• Physical properties are measured without changing the composition

  • Examples: Boiling point, Melting point, Density
• Chemical properties are measured by changing the composition

  • Examples: Acidity, Basicity, Combustion, Oxidation
• The International System of Units (SI Units) includes measurements of mass, length, time, and other physical quantities

  • Examples: Length (metre), Mass (kilogram), Time (second), Electric Current (ampere), Thermodynamic temperature (kelvin), Amount of substance (mole), Luminous Intensity (candela)
• Mass is the amount of matter present and is constant

  • Weight is the force exerted by gravity on an object
  • SI unit of mass is kilograms (Kg)
• SI Unit of Volume is m^3

  • 1 L = 1000 ml = 1000 cm^3 = 1 dm^3
• Density is mass per unit volume

  • SI Unit is kg/m^3
• Temperature can be measured in Degree Celsius (˚C), Degree Fahrenheit (˚F), Kelvin (K)

  • Conversion of Celsius to Fahrenheit: ℉ = 9/5 ℃ + 32
  • Conversion of Celsius to Kelvin (K): K = (˚C) + 273.15
• Significant figures are numbers that include all digits that are certain and a last digit that is uncertain

  • Rules for significant figures are based on the certainty of digits in a number
• Law of conservation of mass states that matter can neither be created nor destroyed

  • Total mass of the reactants is equal to the total mass of the products in any physical or chemical change
• Law of definite proportions states that a chemical compound always contains the same elements combined together in the same proportion by mass
• Law of multiple proportions states that when two elements combine to form more than one compound, the masses of one element that combines with a fixed mass of the other element bear a simple whole number ratio to one another
• Avogadro's law states that equal volumes of gases at the same temperature and pressure contain an equal number of molecules
• Gay-Lussac's law of combining volumes states that when gases combine with each other, they do so in volumes that bear a simple ratio to each other and to the product formed, provided all gases are at the same temperature and pressure
• Dalton's atomic theory:

  • Matter is made up of small indivisible particles called atoms
  • All atoms of a given element are identical in properties, including mass
  • Atoms of different elements differ in mass
  • Atoms are the smallest particles that take part in a reaction
  • Atoms of different elements combine in a fixed ratio to form compounds
  • Atoms can neither be created nor destroyed
• Avogadro number:

  • Avogadro number is the number of atoms present in exactly 12 grams of Carbon-12 isotope
  • Denoted by N_A
  • Value is 6.022 x 10^23
  • 1 gram atomic mass of any element contains Avogadro number of atoms
  • Examples: 16 grams of oxygen, 23 grams of sodium, 14 grams of nitrogen all contain 6.022 x 10^23 atoms
  • 1 gram molecular mass of any substance contains Avogadro number of molecules
  • Examples: 32 grams of oxygen, 28 grams of nitrogen, 58.5 grams of NaCl all contain 6.022 x 10^23 molecules
• Mole:

  • Amount of substance containing Avogadro (6.022 x 10^23) number of particles
  • Denoted by mol
  • Examples: 1 mole of oxygen = 16 grams = 6.022 x 10^23 atoms, 1 mole of nitrogen = 28 grams = 6.022 x 10^23 molecules
• Atomic mass:

  • Atomic mass is the mass of one atom of any element
  • 1 atomic mass unit (amu) = 1/12th mass of one atom of Carbon-12 isotope
  • 1 amu = 1.66056 x 10^-27 kg
• Molecular mass:

  • Sum of atomic masses of all elements present in one molecule of a substance
  • Example: Molecular mass of water (H2O) = 2 x 1.008 + 16.00 u = 18 u
• Percentage composition:

  • Relative measure of the mass of each element present in a compound
  • Mass % element = (Mass of the element / Molar mass of the compound) x 100
  • Example: Calculate the percentage composition of elements in methanol (CH3OH)
• Empirical formula:

  • Represents the simplest whole number ratio of atoms of different elements in a compound
  • Example: Empirical formula of acetic acid is CH2O
• Molecular formula:

  • Represents the actual number of atoms of different elements in a compound
  • Example: Molecular formula of acetic acid is CH3COOH
• Stoichiometry and stoichiometric calculations:

  • Stoichiometry deals with the study of quantities of reactants and products in chemical reactions
  • Balanced Chemical Reaction: Example with methane gas reacting with oxygen to form carbon dioxide and water
• Limiting reagent:

  • The reagent completely consumed in the reaction
  • Excess reagent: The reagent not completely consumed and left over at the end of the reaction
• Some basic concepts of chemistry:

  • Avogadro's law: Equal volumes of gases at the same temperature and pressure contain equal number of molecules
  • Mole fraction: Ratio of number of moles of one component to the total number of moles of all components in a solution
  • Molarity: Number of moles of solute dissolved in one dm^3 or one liter of the solution
  • Molality: Number of moles of solute dissolved in 1000 grams of solvent
• Molarity is defined as the number of moles of solute dissolved in one dm3 or one liter of the solution
• Molality is defined as the number of moles of solute dissolved in one kg of the solvent
• One atomic mass unit is 1.66 x 10^-24 g
• Limiting reagent is a reactant that is completely consumed in a chemical reaction