6.02*10^23 given the symbol Na is the number of particles in one mole of a substance.
Relative atomic mass (Ar)
tells you the number of times an atom of an element is heavier that one-twelfth of an atom of C-12. Ar values have no units.
Relative formula mass (Mr)
is the sum of the relative atomic masses for each of the atom in the formula. for a simple compound it is sometimes called relative molecular mass. Mr values have no units. Eg. CH4= 12+(1*4)=16.
tells you the simplest ratio of atoms of each element in a compound.
tells you the actual number of atoms of each element in a molecule. This may be either the same as or a whole number multiple of the emprical formula.
To work out the amount of substance, use Amount= Mass(g) / Ar
molecule/compound : amount= Mass(g) / Mr
Example 1: (working out the formula from reacting masses)
2.43g of magnesium reacts with 0.20g of hydrogen.What is the formula of magnesium hydride?
Step 1: 2.43g of Mg= 2.43 / 24.3=0.100 moles.
Step 2: 0.20g of H= 0.20 / 1.0= 0.20 moles
Step 3: dividing through by the smallest number of moles (0.1 in this case) gives 1Mg :2H. the empirical formula is therefore MgH2.
Example 2: (working out the percentage by mass of an element)
Calculate the percentage by mass of nitrogen in ammonium sulfate, (NH4)2 SO4.
Step 1: Mr (NH4)2 SO4= 2* (14+1+1+1+1)+32.1+(4*16)=132.1
Step 2: Mass of nitrogen in 1mole of (NH4)2 SO4= 14+14=28
Step 3: % by mass= (mass of an element in 1 mole/ Mr of compund) * 100