Unit One Chemistry – Horsham College Measurement in Chemistry Unit One Chemistry – Horsham College Chapter 6 – VCE 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 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.