The Revision Den

Definitions: Relative atomic mass: The weighted average mass of all the isotopes of an element compared to 1/12th of the mass of a carbon-12 atom. This is the value shown on your periodic table. Relative molecular mass: The weighted average of the masses of the molecules of a compound relative to 1/12th of the mass of a carbon-12 atom. Relative formula mass: This is the weighted average of the masses of the formula units of a substance relative to 1/12th of the mass of a carbon-12 atom. (This is only used in macromolecules such a graphite or diamond). Molar mass: This is the mass of a given substance (element or compound) divided by the amount of substance. Measured in g/mol. Parts per million (ppm): This is the concentration by volume of one part of gas, or by weight of one part of a liquid or solid, per million parts of air or liquid. It is just like a percentage (%), but instead of being out of 100 it is out of 1,000,000. Examples:         1. W...

Definitions: Ionic Equation: An equation that shows the ions involved in the reaction as well as other non-ionic elements/compounds. Ion :  An ion is formed when an atom gains or loses one or more electrons. Ions are charged particles. Ionic compounds are formed from ions. You will be expected to know some ions off by heart, below is a list of the most common ones: Ion Symbol Ammonium NH₄⁺ Carbonate CO₃²⁻ Hydrogen-carbonate     HCO₃⁻ Hydroxide OH⁻ Nitrate NO₃⁻ Phosphate PO₄³⁻ Sulphate SO₄²⁻ Examples: 1. Iron metal reacts with copper(ii)sulphate solution, the products are copper and iron(ii)sulpahte. Find the ionic equation for this reaction. Although not necessary I would recommend you to write down the symbol/stoichiometric equation, as it will help you visualise and balance the equation. In this case:  Fe(s) + CuSO₄(aq) → Cu(s) + FeSO₄(aq) As we can see this equation is already balanced. Next, you have to separate it int...

Definitions: Mole: 1 mole is the amount of substance that contains as many particles as there are in exactly 12 grams of carbon-12. Avogadro's Constant ( L ): This has a value of 6.02 x 10 23 . It is the number of particles in 1 mole. Examples:         1. Calculate the number of molecules in 2 moles of H 2 O. When you are asked for the number of moles, it is pretty straightforward, you simply have to multiply the number of moles by Avogadro's constant ( L ,  6.02 x 10 23 ). In this case: 2 x  6.02 x 10 23  = 12.04 x 10 23  (molecules), or 1.204 x 10 24  (molecules).         2. Calculate the number of molecules in 24 grams of CaCO 3 . You must be careful, as you are given the mass in grams of CaCO 3 , NOT THE NUMBER OF MOLES , so you must convert it to moles using the equation nº of moles, n = mass, m  / molar mass, M r .  M r  of CaCO 3  = 100 Therefore n = 24/100 = 0.24 m...

Definitions: Atom: An atom is the smallest part of an element that can take part in a chemical reaction. Element: An element is a substance that cannot be broken down by chemical means into other substances. Ion: An ion is formed when an atom gains or loses one or more electrons. Ions are charged particles. Ionic compounds are formed from ions. Molecule: A molecule is the smallest part of a covalent compound or element that can exist on its own. Compound: A compound is formed when two or more different elements are chemically bonded together. Empirical Formula: This gives the simplest ratio of atoms of each element in a compound. Molecular Formula: This gives the actual number of atoms of each element in a compound.