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Percentage Formula Mass Sunday, 14 September 2014

Percentage Formula Mass Sunday, 14 September 2014. STARTERS 1. Spot the mistake - ‘When different compounds join together, atoms of the same element are made.’

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Percentage Formula Mass Sunday, 14 September 2014

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  1. Percentage Formula MassSunday, 14 September 2014 STARTERS 1. Spot the mistake - ‘When different compounds join together, atoms of the same element are made.’ [Compounds do not join together, atoms join up to make compounds. Also, if all the atoms are the same in a material, then it is an element, not a compound.] 2. Define - Match these definitions to key words - electron, neutron, proton: Positive particle in the nucleus of an atom = Negative particle that orbits the nucleus of an atom = A sub-atomic particle that carries no charge and is found in the nucleus of the atom =

  2. STARTERS 1. Spot the mistake - ‘When different compounds join together, atoms of the same element are made.’ [Compounds do not join together, atoms join up to make compounds. Also, if all the atoms are the same in a material, then it is an element, not a compound.] 2. Define - Match these definitions to key words - electron, neutron, proton: Positive particle in the nucleus of an atom = Negative particle that orbits the nucleus of an atom = A sub-atomic particle that carries no charge and is found in the nucleus of the atom =

  3. Moles and Percentage formula Mass Additional Chemistry Sunday, 14 September 2014

  4. Learning Outcomes • You should be able to: • Calculate the percentage composition of an element in a compound. • Explain what a mole is (not that kind)!!

  5. What is an Isotope? • Draw C14 and C12 and tell me how they differ?

  6. Examples of isotopes What differs between isotopes? NB. There is also a Carbon 13

  7. Relative Formula Mass • Relative Formula Mass Calculation (Mr)  • Calculate Remember AR = mass of atom • Water H2O MR = mass of formula • H AR = 1 • O AR = 16 • So: 1 + 1 + 16 =18 • Sodium hydroxide NaOH • Na AR = 23 • O AR = 16 • H AR = 1 • So : 23 + 16 + 1 = 40 • Magnesium hydroxide Mg(OH)2 • 24 + 16 + 16 + 1 + 1 = 58

  8. Percentage composition? • Percentage composition is just a way to describe what proportions of the different elements there are in a compound • If you have the formula of a compound, you should be able to work out the percentage by mass of an element in it.

  9. Percentage composition • Example • The formula for sodium hydroxide is NaOH. • It contains three different elements: Na, O and H. • But the percentage by mass of each element is not simply 33.3 per cent, because each element has a different relative atomic mass. • You need to use the Ar (relative atomic mass) values to work out the percentages. • Here is how to do it:……………….

  10. Working out % masas • E.g. What is the percentage by mass of oxygen (O) in sodium hydroxide (NaOH)? • First, work out the relative formula mass (Mr) of the compound, using the Ar values for each element. • Na = 23, • O = 16, so Mr of NaOH is Na + O + H • H = 1. 23 + 16 + 1 = 40 • (You will be given these numbers in the exam.) • Next, divide the Ar of oxygen by the Mr of NaOH, and multiply by 100 to get a percentage. • (16 ÷ 40 ) × 100 = 0.4 × 100 = 40% • So the percentage by mass of oxygen in sodium hydroxide is 40%.

  11. Percentages and Formula • When a mining company is deciding whether to exploit minerals they use percentage composition calculations to see whether it is worth the £££ • We can use relative atomic mass of a compound to work out its percentage composition • When can then use these percentages to calculate the empirical formula egMgO,

  12. Mass of element in question Total mass of compound x100 C2 3.3 Percentages & Formulae • We can use Ar and Mr to work out the percentage of an element in a compound • In MgO, how much is Mg, how much is O? • Magnesium Ar = 24; Oxygen Ar = 16 • Magnesium Oxide Mr = 40 (mass of Mg/total) x100 (24/40) x 100 = 60% • What is the percentage mass of hydrogen in methane, CH4? C=12, H=1 4/16=0.25 X100 = 25%

  13. Al 27 13 e.g. Water Hydrogen Peroxide Molecular Formula H2O H2O2 Empirical Formula H2O HO 9g 27g 35.5g 35.5g C2 3.3 Percentages & Formulae • Working out the formula of a compound from its percentage composition • aka the compounds Empirical Formula – higher tier • If 9g Aluminium reacts with 35.5g Chlorine what is the empirical formula of the compound formed? Simplest whole number ratio: 3xCl, 1xAl = 0.33 of a mole = 1 mole Al: Cl: AlCl3 Mass of 1 mole Mass of 1 mole

  14. Learning Outcomes • You should be able to: • Calculate the percentage composition of an element in a compound. • Explain what a mole is (not that kind)!! • Say what we mean by yield • Say what atom economy is and why its important • Identify a reversible reaction

  15. Starter • Re-cap on Mr – relative formula mass and percentage formula mass • What is the percentage of hydrogen (H) in ethanol – C2H5OH • Whole = 12+12+1+1+1+1+1+16+1 = 46 • Part = hydrogen 1+1+1+1+1+1 = 6 • Part/whole x100 = 6/46 x100 = 13%

  16. So what is a mole? • The relative formula mass of a substance, shown in grams, is called one mole of that substance. • So one mole of carbon monoxide has a mass of 28g, • carbon monoxide (CO). • The Ar of carbon is 12 and the Ar of oxygen is 16. • So the Mr of carbon monoxide is 12 + 16 = 28. • So one mole of CO has a mass of 28g • One mole of sodium oxide has a mass of 62g. • Sodium oxide (Na2O). • The Ar of sodium is 23 and the Ar of oxygen is 16. • So the Mr of sodium oxide is (23 x 2) + 16 = 62. • So one mole of Na2Ohas a mass of 62g

  17. Clickview video • Explain a bit more – ignore the cheesy Australian accent! • Moles (first 15 mins)

  18. Moles • Basically • A mole is the same as the Ar of an atom or the Mr of a compound in grams!!! • Magnesium's Ar = 24 • So one mole of Mg is 24 g • Magnesium Oxide’s (MgO) Mr = 24+16 = 40 • So one mole of MgO is 40 g • EASY! It’s just the smallest version of an atom or molecule. • Do sheet calculating mass of a mole

  19. Questions • Use your question sheet from before and work out Q) 2 and 4

  20. The relative formula mass of a substance, in grams, is known as one mole of that substance. Or 12g

  21. Atom Economy!! • An economical atom???....? • Not far off. This is when we look at the most economic way of making a substance • Because….when we react substances together to make something new, we lose some of the substances along the way. Think of making a cake – not all the mixture makes it into the oven!

  22. Atom economy • So…. A + B = C • Whilst mixing some of A might be left in the mixer and therefore some of B will be wasted as it won’t have enough of A to mix with….etc. So we never get 100% yield. It may be reversible so it’s converting back • Yield = is how much of our product we get compared to out reactants.

  23. What is a reversible reaction • This is the last bit on equations! I Promise! • Using the textbooks you are going to investigate reversible reactions and look at an example of one. • Use page 126 in the purple textbooks to find out what reversible reactions are. • Use the key points at the bottom of page 127 to write a paragraph on reversible reactions – imagine you were having to explain it to a year 8 so phrase it simply

  24. The Harbour Process • Making Ammonia • You are going to produce a factsheet explaining how we use these reversible reactions to make ammonia. • You need to include: • What we use ammonia for • What we make ammonia from? • The conditions needed to make ammonia • A diagram showing how the process works • The equation for ammonia (note that it is reversible)

  25. Learning Outcomes • You should be able to: • Calculate the percentage composition of an element in a compound. • Explain what a mole is (not that kind)!! • Say what we mean by yield • Say what atom economy is and why its important • Identify a reversible reaction

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