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Discuss in pairs What is an isotope?
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Learning Objectives Grade C/D Recall what an isotope is.
Define relative atomic mass and relative formula mass. Grade A/B Calculate RAM of an element using information on the relative abundances of its isotopes. Calculate the RFM of a compound and the percentage by mass of an element in a compound.
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These are the mass numbers of atoms of these elements
What is the mass of... 1 atom of carbon? 1 atom of lithium? 2 atoms of mercury? 5 atoms of neon? These are the mass numbers of atoms of these elements
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ATOMIC MASS Atoms are so small that we can’t use units like grams.
Instead, the mass of all atoms is compared with the mass of an atom of a carbon-12 isotope (the carbon-12-scale). These atoms weigh exactly 12 units. If we compare an atom of the most common isotope of magnesium with carbon-12, it has exactly double the mass (magnesium-24). This is the relative isotopic mass. An atom of the most common isotope of hydrogen weighs one-twelfth of carbon-12 (1 unit)
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Relative Atomic Mass (Ar)
The Ar of an element is the weighted average mass of the isotopes of the element. It is always measured on a scale on which a carbon-12 atom has a mass of 12 units. As it is the average, it is often not a whole number.
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How were the Ar‘s of each element worked out?
If you take a sample of an element then you are likely to have a mixture of atoms of different isotopes of the element. E.g. In a sample of chlorine, some atoms have a mass of 35 and others have a mass of 37. However, this doesn’t mean that the average Ar is 36. A typical sample contain 75% chlorine-35 and 25% chlorine-37 - For 100 atoms of Cl, the mass is: (75 x 35) + (25 x 37) = 3550 atom of Cl has an average mass of 35.5
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Example A magnesium sample contains 78.6% Mg-24, 10.1% Mg-25 and 11.3% Mg-26. Assume there are 100 atoms in the sample, calculate the Ar of magnesium to 1 decimal place. Total mass = (78.6 x 24) + (10.1 x 25) + (11.3 x 26) Ar = /100 = 24.3
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You could also be asked to use a graph like this one to calculate the Ar of an element
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Relative Formula Mass The relative masses of all the atoms in a compound added together. eg Copper Sulphate, CuSO4 contains: 1 x Cu atom (mass 64) 1 x S atom (mass 32) 4 x O atoms (mass 4 x 16) Total = = 160 Remember: No units!!!
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Find the RFM of the following compounds
NH3 (NH4)2SO4 NH4Cl Mg(NO3)2 H2SO4 Ca(NO3)2 HNO3 Mg(OH)2 CaCO3 Ca(OH)2 17 53.5 98 63 100 Brackets: 132 148 164 58 74
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% of elements in a compound
Malachite (copper ore) has the formula CuCO3 What is the % by mass of copper in this ore? Total mass = (16 x 3) = 124 64 of the 124 is copper (64/124) x 100 = 51.6%
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Solve the following: What is the % by mass of copper in copper oxide (CuO)? What is the % by mass of water in CaSO4.2H2O? What is the % by mass of sulphur in sulphuric acid (H2SO4)?
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Learning Objectives Grade C/D Recall what an isotope is.
Define relative atomic mass and relative formula mass. Grade A/B Calculate RAM of an element using information on the relative abundances of its isotopes. Calculate the RFM of a compound and the percentage by mass of an element in a compound.
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