Relative Formula Mass (RFM) or Molar Mass Found by adding all of the individual RAM’s together in one formula unit of an ionic compound.
Avogadro’s number and the mole concept • In chemistry, amounts of substances are measured in moles (mols). • The mole is a standard number of particles (atoms, ions or molecules) and can be defined as, the amount of anysubstance that contains the same number of particles, as there are C 12 atoms in 12.00g of the C 12 isotope. • The actual number of particles in a mole, known as the Avogadro constant or number is found to be 6.022 x 10 23 particles per mole and has the unit, mol-1. E.g. One mole of atoms = 6.02x 10 23 atoms. • The average Relative Atomic Mass (RAM) of each element is given on the periodic table. The figure shows the average mass of one mole of atoms of that particular element. • This leads to the relationships below.
Limiting Reactant When all the reactants in a chemical reaction are completely consumed, i.e. they are all converted to products, then the reactants are said to be in stoichiometric proportions. On other occasions it may be necessary to ensure that only one particular reactant is completely used up. This is achieved by using an excess of all the other reactants. The reactant that is completely consumed is called the limiting reagent and it, is what determines the quantities of products that form.
Percentage yield In all chemical syntheses the yield of the product will be less than 100%. The % yield is given as The yield is usually less than 100% since the reactants are often not pure, some of the product is lost during purification, the reaction may be reversible and/or side reactions may give byproducts.
If we know the number of moles of a substance that is present in a reaction and we know a balanced chemical equation, (i.e. we know the reacting ratio), it is possible to calculate the moles of another substance present in the equation. Use this method 1. Write a correct and balanced equation. 2. Find the number of moles present by using a moles relationship for one substance. 3. Use the stoichiometric coefficients in the equation to find the reacting ratio of the moles. Use this relationship to find the number of moles of the unknown substance. 4. Re-apply a moles relationship for the unknown substance.
Chemical reactions are often carried out between substances that are in solution (dissolved in a solvent, usually water). The concentration of a solution, can be measured in terms of the number of grams of the solute (solid) that has been dissolved in a particular volume of the solvent (usually water), or in terms of the number of moles of the solute in a particular volume of the solvent. Typical units are g dm -3 or g/dm 3 or mol dm -3 or mol/dm 3 or mol/L or mol L -1 The method of expressing the concentration of a solution in mol L -1 is the most common and useful, and is referred to as Molarity (M).
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