SC9a Masses And Empirical Formula Flashcards
what is the relative formula mass of a substance?
sum of the relative atomic masses (Ar) of all the atoms or ions in its formula.
Calculate the Mr of:
Mg(NO3)2
148
what is the empirical formula?
simplest whole number ratio of atoms or ions of each element in a substance.
how can the empirical formula be calculated?
This can be calculated from the masses of the reactants used to make it.
calculate the empirical formula if 10.0g Calcium reacts with 17.8g Chlorine:
mass ca - 10 cl - 17.8
Ar 40 35.5
mass / Ar 0.25 0.5
divide by smallest number ca - 1 cl - 2
empirical formula CaCl2
what numbers is the empirical formula written in?
whole numbers
what is the molecular formula?
actual number of atoms of each element in one molecule.
molecular formula to empirical formula C4H8
CH2
how to find the molecular formula from the empirical formula?
- find Mr
- divide formula mass by Mr
- answer times the empirical formula
The empirical formula of Hydrocarbon X is CH2. The formula mass of Hydrocarbon X is 70. Determine the molecular formula of Hydrocarbon X.
Mr = 14
70/14 = 5
CH2 x 5 = C5H10
Method for finding the empirical formula by experiment.
~Find the mass of a crucible and lid.
~Clean the magnesium ribbon with emery paper.
~Coil the magnesium ribbon so that it fits in the crucible. ~Find the mass of the crucible, lid and magnesium ribbon.
~Set up the apparatus as shown in the diagram.
~Heat the crucible strongly. Occasionally lift the lid with a
pair of tongs.
~Continue heating until all of the magnesium
has completely burned.
~Allow the crucible to cool to room temperature.
~Find the mass of the crucible, lid and magnesium oxide.
