Unit 3: Quantitative Chemistry

Unit 3: Quantitative Chemistry

1. Relative Formula Mass

   • The relative mass is just the relative atomic masses of all of the elements in the molecular formula added together.
   • Percentage of mass of an element in a compound =
     • Ar x number of atoms of that element ____________________________________________________ x 100 Mr of the compound

2. The Mole and Mass

   • One mole of any substance is just the amount of the substance that contains an Avogadro number of particles.
     • This means that there are the same number and same type of atoms on each side of the reaction equation.
     • Because of this, no mass is lost or gained, we say the mass is conserved during a reaction.
     • E,g. 2LI + F2 = 2LiF
   • Avogadro Constant = 6.02 x 10↑23
   • E,g. Carbon has an A of 12g. So one mole of carbon weighs exactly 12g.
   • Number of Moles =
     • mass in g (of an element or compound ---------------------------------------------------------------------------- Mr (of the element or compound
   • In a chemical reaction, mass is always conserved.
   • During a chemical reaction, no atoms are destroyed and no atoms are created.
   • if a mass seems to change, there's normally a gas involved..

3. The Mole and Equations

   • The numbers in chemical equations mean different things.
   • You can balance equations using reacting masses.
   • Reactions stop when one reactant is used up.
   • Volume of gas =
     • Mass of gas ------------------ Mr of gas