Unit One Chemistry – Horsham College

Chapter 6 – VCE Chemistry

Measurement in Chemistry

Atomic Weight• As chemists gradually accumulated data on

elements, they found that elements always combined in particular proportions by mass.

• From this early chemists constructed tables of atomic weights – how heavy the atoms of each element were in relation to each other.

• Berzelius (1826) produced a table of weights in relation to the lightest element hydrogen taken as 1.

• The modern method is to use the standard as the mass of the carbon –12 isotope taken as 12 exactly.

Introducing the Mole

• The mole is a unit used to calculate the very large numbers of particles used in chemistry.

• A mole is defined as the amount of substance that contains a many particles as there are in 12g of carbon-12 isotope.

• The word mole originates from the German word molekulargewicht when translated means molecular weight.

Avogadro’s Number

• The number of particles in 12g of carbon is 6.02 x 1023.

• This is called Avogadro’s number (NA)

• 1 mol of any substance contains 6.02 x 1023 particles.

SI Units (Le Système international d'unités) Name Symbol Quantity

metre m length

kilogram kg mass

second s time

ampere A electric current

Kelvin K thermodynamic temperature

mole mol amount of substance

candela cd luminous intensity

Why the Large Number?

• Avogadro’s number is based on the mass of a Carbon-12 atom.

• 12g of Carbon has 1 mole of Carbon atoms.• 1g of Hydrogen has 1 mole of Hydrogen atoms.• 16g of Oxygen has 1 mole of Oxygen atoms.

Questions 6.1

• Complete questions 1-3 on P. 115 of Nelson VCE Chemistry.

Particles in ‘Packages’

• In chemistry, substances are often found in packages such as molecules or compounds.

• Lets look at sucrose with the formula C12H22O11.

• In total there are 45 atoms in a sucrose molecule. Therefore to calculate the total number of atoms you must times the final answer by 45.

• For individual elements eg. Carbon, you must times the total number of atoms by 12.

Questions 6.2

• Complete questions 1-9 on P. 119 of Nelson VCE Chemistry.

Molar Mass

• The molar mass of an element is defined as the mass of 1 mol of an element expressed in grams per mole (g mol-1).

• Has the symbol M (Does not have a subscript because it has an actual mass)

Relative Mass of Ionic Compounds and Ions

Compounds;• The relative formula mass used to calculate the

mass of a compound.Ions;• Ions contain charged particles.• These particles have negligible mass relative to

the atoms present and are therefore not counted in the molar mass.

Questions 6.3

• Complete questions 1-6 on P. 122-3 of Nelson VCE Chemistry.

Percentage Composition by Mass

• Atoms in an ionic lattice are arranged a fixed ratio.

• There masses will also be in a fixed ratio.

Questions 6.4

• Complete questions 1-6 on P. 126-7 of Nelson VCE Chemistry.

Relating Number of Moles to Mass

• The relationship between the number of moles to mass is;

• This can be rearranged to find the mass based on number of moles;

Determining Empirical Formulas

The empirical formula gives the simplest whole number ratio of the atoms present in a compound.

Steps;• Write down the chemical symbols• Assume that the mass if 100g. Convert % into grams.• Convert masses in moles (g/Ar)• Divide each by smallest number of moles.• If required, multiply by common number to achieve whole numbers.

Questions 6.5

• Complete questions 1-5 on P. 129 of Nelson VCE Chemistry.

Degree of Hydration of Hydrated Salts

• A hydrated salt contains a certain proportion of water molecules, which surround the ions in the solid crystals.

• Eg CuSO45H2O has a degree of hydration of 5. • If all the water of hydration is removed, the salt is

said to be anhydrous. • We can treat water as if it is an element and

calculate empirical formula to determine degree of hydration.

Questions 6.6

• Complete questions 1-6 on P. 133 of Nelson VCE Chemistry.

Review Questions

• Complete all questions on P. 135 of Nelson VCE Chemistry.