Atomic masses L.O.:  Define the terms relative isotopic mass and relative atomic mass, based on the 12C scale;  Calculate the relative atomic mass of an element given the relative abundances of its isotopes;  Use the terms relative molecular mass and relative formula mass and calculate values from relative atomic masses.

HW: pre-work on mole Read worksheet and answer Q1-2Q1-2

Relative atomic mass is the mass of an atom of an isotope compared with one-twelfth of the mass of an atom of carbon-12

ISOTOPES - CALCULATIONS There are two common isotopes of chlorine. Calculate the average relative atomic mass of chlorine atoms ProtonsNeutrons% 35 Cl Cl

ISOTOPES - CALCULATIONS There are two common isotopes of chlorine. Calculate the average relative atomic mass of chlorine atoms Out of every 100 atoms 75 are 35 Cl and 25 are 37 Cl Average = (75 x 35) + (25 x 37) = ProtonsNeutrons% 35 Cl Cl

1.1 Exercise 2

Worked examples: 1.What is the relative atomic mass (A r ) of a mixture of 90% hydrogen-1 ( 1 H) and 10% hydrogen-2 ( 2 H)? 2.What is the relative atomic mass (A r ) of a mixture of 75% chlorine-35 ( 35 Cl) and 25% chlorine-37 ( 37 Cl)? Calculate the A r of 1.43% 16 O and 57% 18 O 2.14% 40 Ca, 26% 42 Ca and 60% 46 Ca

Relative molecular mass, Mr, is the weighted mean mass of a molecule compared with one- twelfth of the mass of an atom of carbon-12

Relative molecular mass, M r This is the total mass of a given molecule calculated by adding together the relative atomic masses (A r ) of all of its atoms Worked examples: M r of H 2 O: 2x 1 (H) + 1x 16 (O) = 18 M r of HCl: 1x 1 (H) + 1x 35.5 = 36.5

Work out the relative molecular masses of these molecules: CH 4 CO 2 HBr H 2 SO 4 CH 2 Cl 2

Work out the relative molecular masses of these molecules: CH 4 CO 2 HBr H 2 SO 4 CH 2 Cl 2

Relative formula mass Very large compounds such as NaCl and SiO 2 are too big to work out the mass of the whole structure, so we only work out the basic unit Worked examples: NaCl and SiO 2

Relative formula mass is the weighted mean mass of a formula unit compared with one- twelfth of the mass of an atom of carbon-12

Jan 11, Q1

Atomic masses L.O.:  Define the terms relative isotopic mass and relative atomic mass, based on the 12C scale;  Calculate the relative atomic mass of an element given the relative abundances of its isotopes;  Use the terms relative molecular mass and relative formula mass and calculate values from relative atomic masses.