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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.