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# 3 na calculating_mr

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### 3 na calculating_mr

1. 1. Chapter 7 Relative Masses of Atoms and Molecules
2. 2. contents <ul><li>You will learn about: </li></ul><ul><li>Relative atomic mass </li></ul><ul><li>Relative molecular mass </li></ul><ul><li>The formula of a compound </li></ul>
3. 3. Objectives <ul><li>At the end of the lesson, students should be able to: </li></ul><ul><ul><li>define relative atomic mass , A r </li></ul></ul><ul><ul><li>define relative molecular mass , M r </li></ul></ul><ul><ul><li>calculate relative atomic mass, relative molecular mass and relative formula mass </li></ul></ul>
4. 4. Introduction Atoms are soooo small and their masses are so small! Is there a way to measure them conveniently??? Since atoms are so small, let’s compare them to each other on the atomic scale.
5. 5. We can compare every element to H or 1/12 of C-12
6. 6. relative atomic mass The lightest atom is the hydrogen atom. As one nitrogen atom is 14 times heavier than a hydrogen atom, nitrogen is said to have a relative atomic mass of 14. N H H H H H H H H H H H H H H
7. 7. relative atomic mass Relative masses is about comparing the masses of atoms and molecules. relative atomic mass of an element <ul><li>is the average mass of one atom of the element when compared with 1/12 of the mass of an atom of carbon-12 </li></ul><ul><li>symbol is A r </li></ul><ul><li>has no units </li></ul>Isotope carbon-12 was chosen (instead of hydrogen) for comparing the masses of atoms as it is in abundance and has isotopes. relative atomic mass of an element = average mass of one atom of the element mass of 1/12 of an atom of carbon-12
8. 10. relative atomic mass of an element Relative atomic masses of common elements: relative atomic mass Si 28 silicon O 16 oxygen N 14 nitrogen Mg 24 magnesium Na 23 sodium Al 27 aluminium P 31 phosphorus He 4 helium S 32 sulphur carbon hydrogen Elements 12 1 Relative Atomic Mass C H Symbol
9. 11. relative atomic mass of an element Relative atomic masses of common elements: relative atomic mass Cu 64 copper Zn 65 zinc Br 80 bromine I 127 iodine K 39 potassium Ca 40 calcium C l 35.5 chlorine Fe 56 iron lead Elements 207 Relative Atomic Mass Pb Symbol
10. 12. Question <ul><li>What is the difference between nucleon number and relative atomic mass, A r ? </li></ul>Nucleon number represents the total number of proton and neutron in an atom but A r takes into account the presence of isotopes.
11. 13. Question <ul><li>Why are some A r values not whole numbers? </li></ul><ul><li>Example: A r of Cl is 35.5. </li></ul><ul><li>Because most naturally occurring elements contain a mixture of isotopes. </li></ul><ul><li>Eg. 75% of chlorine-35 </li></ul><ul><li>25% of chlorine-37 </li></ul><ul><li>To calculate A r of Cl: </li></ul><ul><li>A r of Cl = (75/100 x 35) + (25/100 x 37) </li></ul><ul><li> = 26.25 + 9.25 </li></ul><ul><li> = 35.5 </li></ul><ul><li>The relative atomic mass of chlorine is 35.5. </li></ul>
12. 14. Question <ul><li>The mass of one atom of an element X is 1 1/3 that of carbon. What is its relative atomic mass? </li></ul><ul><li>What is the relative atomic mass of element Y , given that it is four times the mass of a carbon-12 atom? </li></ul><ul><li>1. Relative atomic mass of X = (4/3) x 12 = 16 </li></ul><ul><li>2. Relative atomic mass of Y = 4 x 12 = 48 </li></ul>
13. 15. relative molecular mass relative molecular mass of a molecule <ul><li>is the average mass of one molecule of a substance when when compared with 1/12 of the mass of an atom of carbon-12 </li></ul>relative molecular mass of a molecule = average mass of one molecule of a substance mass of 1/12 of an atom of carbon-12 <ul><li>symbol is M r </li></ul><ul><li>has no units </li></ul><ul><li>is the sum of the relative atomic masses of all the atoms in the molecule </li></ul><ul><li>for ionic compounds, relative formula mass is used </li></ul>
14. 16. relative molecular mass relative molecular mass of a molecule <ul><li>to find relative molecular mass of carbon dioxide (CO 2 ), </li></ul>relative atomic mass of carbon 2 x relative atomic mass of oxygen relative molecular mass of CO 2 = 12 = 2 x 16 = 32 (total) = 44
15. 17. relative molecular mass relative molecular mass of a molecule <ul><li>to find relative molecular mass of sugar (C 12 H 22 O 11 ), </li></ul>12 x relative atomic mass of carbon 22 x relative atomic mass of hydrogen 11 x relative atomic mass of oxygen relative molecular mass of C 12 H 22 O 11 = 12 x 12 = 144 = 22 x 1 = 22 = 11 x 16 = 176 (total) = 342
16. 18. Relative molecular mass of some molecules 1 + 35.5 = 36.5 1H; 1Cl HCl Hydrogen chloride 32 + 16 x 2 = 64 1S; 2O SO 2 Sulfur dioxide 12 + 16 x 2 = 44 1C; 2O CO 2 Carbon dioxide 1 x 2 + 16 = 18 1O; 2H H 2 O Water 14 x 2 = 28 2N N 2 Nitrogen Calculating M r Atoms in formula Formula Substance
17. 19. relative formula mass relative formula mass of ions/ionic compounds <ul><li>Ionic compounds do not contain molecules </li></ul><ul><li>Sum of relative atomic masses of atoms called relative formula mass </li></ul><ul><li>Also given the symbol M r </li></ul>= 12 = 3 x 16 = 48 (total) = 60 relative atomic mass of carbon 3 x relative atomic mass of oxygen relative formula mass of CO 3 2-
18. 20. relative formula mass relative formula mass of ions/ionic compounds <ul><li>Relative formula mass of ammonium ion, NH 4 + </li></ul>relative atomic mass of nitrogen 4 x relative atomic mass of hydrogen relative formula mass of NH 4 + = 14 = 4 x 1 = 4 (total) = 18
19. 21. Relative formula mass of ionic compounds 40 + 12 + 16 x 3 = 100 1Ca;1C;3O CaCO 3 Calcium carbonate 23 x 3 + 31 + 16 x 4 = 164 3Na;1P;4O Na 3 PO 4 Sodium phosphate 56 + 32 + 16 x 4 + 7(1 x 2 + 16) = 278 1Fe;1S;4O;7H 2 O FeSO 4 .7H 2 O Iron (II) sulphate crystals 56 + 3 x 35.5 = 162.5 1Fe;3Cl FeCl 3 Iron (III) chloride 24 + 32 + 4 x 16 = 120 1Mg; 1S;4O MgSO 4 Magnesium sulphate Calculating M r Atoms present Formula Name
20. 22. Question <ul><li>Use the Periodic Table to answer the following questions. </li></ul><ul><ul><li>Four nitrogen atoms have the same mass as one atom of the element X. Identify X. </li></ul></ul><ul><ul><li>How many calcium atoms have about the same mass as one mercury atom? </li></ul></ul><ul><ul><li>a) 4 x 14 = 56 </li></ul></ul><ul><ul><li>The A r of X is 56 and X is iron, Fe. </li></ul></ul><ul><ul><li>b) A r of mercury = 201 </li></ul></ul><ul><ul><li>A r of calcium = 40 </li></ul></ul><ul><ul><li>No. of calcium atoms that have about the same mass as a Hg atom = 5 </li></ul></ul>