Have you ever tried counting the number of rice particles in your bowl?

Relative Masses of Atoms and Molecules

Masses of Particles in Chemistry the mass of a hydrogen atom is g very small values and very inconvenient to write so, scientists do not use these actual masses in their calculation

Relative Masses scientists overcome this by comparing the masses of atoms with the mass of a carbon-12 atom it is a ratio and has no unit

Relative Atomic Mass The relative atomic mass (A r ) of an element is the average mass of one atom of the element when compared with of the mass of an atom of carbon-12. NO UNITS!

Relative Molecular Mass many elements and compounds exist as molecules the mass of a molecule is found by adding the relative atomic masses of all atoms in the molecule this mass is known as the relative molecular mass (M r )

Relative Molecular Mass The relative molecular mass (M r ) of a molecule is the average mass of one molecule of the substance when compared with of the mass of an atom of carbon-12. NO UNITS!

Relative Formula Mass ionic compounds consist of ions, not molecules instead of relative molecular mass, we use the term relative formula mass the relative formula mass of an ionic compound is found by adding the relative atomic masses of all the atoms in the formula

Let’s try! TB P137 Q2 Calculate the relative molecular/formula masses of the following substances. a. H 2 O b. NH 3 c. O 2 d. CaCO 3 e. H 2 SO 4 f. Cu(NO 3 ) 2 g. (NH 4 ) 2 SO 4 h. CuSO 4.5H 2 O

Percentage Composition the percentage composition of an element in a compound can be calculated from: the formula, and the relative atomic masses of its elements percentage of an element in a compound = number of atoms of the element in the formula x A r of the element M r of the compound x 100%

Calculating the mass of an element in a compound mass of an element in a compound = number of atoms of the element in the formula x A r of the element M r of the compound x mass of the sample

Calculating the mass of water in a compound hydrated compounds are compounds that contain water (“water of crystallisation”) crystals are dry because water is part of the crystal structure mass of water in a compound = number of water molecules in the formula xM r of H 2 O M r of the compound x mass of the sample

Let’s try! TB P140 Q2 Calculate a. the percentage of calcium and oxygen in calcium carbonate, CaCO 3, and b. the percentage of oxygen in CuSO 4.5H 2 O

Let’s try! TB P140 Q3 Calculate the mass of a. calcium in 25 g of calcium carbonate, CaCO 3, and b. chlorine in 27 g of copper(II) chloride, CuCl 2