1. kzaman

2. Mass of a Hydrogen atom = Mass 1 proton + mass of 1 electron = 1.67X10 -24 + 9.1x10 -28 = 1.67x10 -24 g or equal to, 0.000 000 000 000 000 000 000 0017 g electron e Proton p Neutron n charge (q) - 1.6 x 10 -19 +1.6x10 -19 0 Relative charge -1 +1 0 mass(g) 9.1x10 -28 1.67X10 -24 1.67X10 -24 mass(amu) 0.00055 u 1.007825 u 1.008665 u Relative mass 0.0005 (0) 1.0007 (1) 1.0086 (1)

3. Mass of a methane molecule, CH 4 = 0.000 000 000 000 000 000 000 022 4 g 2.24 x 10 -23 g

4. Learning outcomes: A student is able to: State the meaning of relative atomic mass based on carbon-12 scale, State the meaning of relative molecular mass based on carbon-12 scale, State why carbon-12 is used as a standard for determining relative atomic mass and relative molecular mass, Calculate the relative molecular mass of substances

5. In practice, chemist do not use these actual masses of atoms and molecules in their calculations. They use relative masses instead. When chemist first started comparing masses of different atoms, they compared each of them with the mass of one hydrogen atom. Relative atomic mass = mass of one atom of an element mass of one atom of hydrogen

6. Electric field Reflection chamber to vacuum pump Gas inlet Ion source Mass spectrometer Y H a b

7. H X H The mass of a X atom = The mass of 2 hydrogen atoms Relative atomic mass of X = 2 What does it mean? The mass of a hydrogen atom = 2 The mass of one X atom An X atom is 2 times heavier than a hydrogen atom X H H

8. The mass of a lithium atom = The mass of 7 hydrogen atoms Relative atomic mass of lithium = 7 The mass of a hydrogen atom = 7 The mass of a lithium atom Li H H H H H H H Li H H H H H H H

9. kzaman C The mass of a carbon atom = The mass of 12 hydrogen atoms or 12 x the mass of a Hydrogen atom Relative atomic mass of carbon = 12 The mass of a hydrogen atom = 12 The mass of a carbon atom The mass of one hydrogen atom = 1 of the mass one 12 C atom. 12 H H H H H H H C H H H H H H H H H H H H

10. In 1991, chemists agreed to compare the masses of atoms with the mass of a carbon atom . Carbon consists of more than one isotope. Carbon-12 was chosen since the isotope has the highest relative abundance among carbon isotopes. Relative atomic mass of an atom is defined by IUPAC as; Relative atomic mass = mass of one atom of an element 1 x mass of one atom of 12 C 12

11. If the atomic mass ratio of an atom of X to 12 C = 0.75, then Mass of one atom of X Mass of one atom of 12 C = 0.75 Mass of one atom X = 0.75 x mass of one atom of 12 C Relative atomic mass of X = 0.75 x mass of one atom of 12 C 1 X mass of one atom of 12 C 12 = 9.0

12. 12 6 C 24 12 Mg Relative atomic mass of Mg = 24 The mass of 1 magnesium atom 1 x the mass of 1 12 6 C atom 12 = 24 The mass of 1 magnesium atom = 24 x 1/12 x the mass of 1 12 6 C atom = 24x 1/12 x 12 = 24 12 6 C 24 12 Mg 12 6 C 12 6 C

13. How many oxygen atoms have equal weight as 4 carbon atoms? [ Relative atomic mass ; oxygen = 16 ; carbon = 12] N x mass of 1 oxygen atom = 4 x mass of 1 carbon atom N = 4 x 12 16 = 3 C C C C O O O C C C C O O O

14. How many times is the mass of 20 neon atoms is heavier than 10 carbon atoms? 20 x mass of 1 neon atom = 20 x 20 10 x mass of 1 carbon atoms 10 x 12 = 3.32 The sum of mass of 20 neon atoms is 3.32 times heavier than the sum of mass of 10 carbon atoms

15. kzaman Relative molecular mass of substance X = The mass of a molecule of substance X 1/12 x the mass of 1 atom of 12 6 C The mass of a water molecule is the sum of mass of all atoms of each element in a water molecule The mass of a water molecule 1/12 x the mass of 1 12 6 C atom Relative molecular mass of water = 1 1 16 Mass of a water molecule = 1u + 1u + 16 u = 18u = 18 u 1 x12 u 12 = 18 The relative molecular mass of a substance is the sum of relative atomic mass of all atoms of each element in the molecule A water molecule is 18 times heavier than 1- twelfth of the mass of a Carbon – 12 atom. H H O

16. Calculate the relative molecular mass of the following compounds 6(12)+12(1)+6(16) = 180 C 6 H 12 O 6 Glucose 2(12)+4(1)+2(16) = 60 CH 3 COOH Ethanoic acid 2(1)+32+4(16) = 98 H 2 SO 4 Sulphuric acid 12+2(16) = 44 CO 2 Carbon dioxide Relative molecular mass Molecular formula Compounds

17. For ionic compounds, the term relative molecular mass is replaced by relative formula mass 65+ 2[14+3(16)] =189 Zn (NO 3 ) 2 Zink nitrate 56 + 3[16 + 1] =107 Fe (OH) 3 Iron(III) hydroxide 64+ 32+4(16) +5[2(1)+16] = 250 Cu SO 4 .5H 2 O Copper sulphate (hydrated) 2[14+4(1)] +32+4(16) = 132 (NH 4 ) 2 SO 4 Ammonium sulphate 2(23) +32+4(16) = 142 Na 2 SO 4 Sodium sulphate Relative formula mass Empherical formula Compound