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Relative Masses - A r and M r

Relative Masses - A r and M r. Relative Atomic mass A r is the mass of the atom when compared to Carbon. The mass of Carbon is defined as 12.00. This value can be found on the periodic table

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Relative Masses - A r and M r

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  1. Relative Masses - Ar and Mr Relative Atomic mass Ar is the mass of the atom when compared to Carbon. The mass of Carbon is defined as 12.00. This value can be found on the periodic table Relative Molar mass Mr is the combined mass of all the atoms in a molecule. It is found by adding the masses of all the atoms in a molecule.

  2. Calculating Molecular mass When calculating molecular mass we always set it out in a proscribed way. Eg. Mass of glucose Mr(C6H12O6) = [6Ar(C)] + [12Ar(H)] +[6Ar(O)] = (6 x 12) + (12 x 1) + (6 x 16) = 180

  3. Calculate the molecular mass of the following • MgCl2 • H2SO4 • N2 • C2H4 • Al(OH)3 • Na2CO3

  4. The Mole (N) • A mole is the number of carbon atoms in 12.000g of carbon-12 • 1g of Hydrogen and 12g of Carbon contain the same number of atoms. • For each element in Argrams of that element there is one mole of atoms • One Mole = 6.02 x 1023 • It is also called Avogadro’s number

  5. Molar Mass (M) • The mass of one mole of something is called its molar mass, (M) • Eg. Ar(S) = 32 M(S) = 32g mol-1

  6. Molar calculations To figure out the number of moles in a substance we use the formula or It can be rearranged to give the formulas

  7. Example What is the mass of 5 moles of Carbon dioxide? M(CO2) = 44g mol-1 n(CO2) = 5 mol M(CO2) = 44g mol-1m(CO2) = ? m = nM = 5mol x 44g mol-1 = 220g

  8. Practise calculations Calculate the mass of 1mol O2 2 mol SO2 ½ mol CO2 How many moles are there in 190g of MgCl2 0.28g of Fe 56g N2

  9. Experiment Read page 81 Visit each of the stations around the room, and while at each station fill in the row in the data table on pg 83

  10. A mole of a molecule can contain more than one mole of each of the constituent atoms i.e. 1 mol of H2O contains 1 mol of Oxygen and 2 mol of Hydrogen Eg. Moles of Cl- in 95g MgCl2 M(MgCl2) = 95g mol-1 m(MgCl2) = 95g n(MgCl2) = m/M = 95g/95g mol-1 = 1mol MgCl2 in 95g MgCl2 n(Cl-) in MgCl2 = 2x n(MgCl2) = 2x1mol = 2 mol

  11. How many moles of Al3+ in10.2g Al2O3 How many moles of O in 98g of H2SO4 How many moles of N in 56g of N2

  12. Using the mole A balanced equation tells us the number of moles of each reactant and product. Eg 2H2(g) +O2(g)  H2O(l) The number of moles of H2 is twice that of O2 The number of moles of O2 is half that of H2 The number of moles of H2 is equal to that of H2O

  13. Calculating mole ratios Decide which is the known and which is the unknown substance Write the mole ratio of unknown to known with unknown on top using the coefficients in the balanced equation Put the values into the expression to calculate the volume of the unknown substance

  14. Using Mole Ratios: example 2C2H6(g) +7O2(g) 4CO2(g) +6H20(l) How many moles of oxygen are required to burn 5 moles of ethane? n(C2H6)= 5mol n(O2)= ?

  15. Mole calculations Where the number of moles of one reagent is known the number of the other can be calculated. The basic method for solving mole calculations is • Write a balanced equation for the reaction • Calculate the number of moles of the known reagent • Use the balanced equation to calculate the number of moles of the unknown reagent • Calculate the mass of the unknown reagent

  16. Practise questions Pb(NO3)2(aq) +2NaBr(aq) PbBr2(s) + 2 NaNO3(aq) How many moles of Pb(NO3)2 react with 0.060 moles of NaBr? How many moles of PbBr2 would be formed from 2.8x10-3mol of Pb2+(aq)? How many moles of NaBr would have reacted to make 0.4mol of PbBr2? How many moles of Pb(NO3)2 would have reacted to make 0.5mol of NaNO3?

  17. Al(OH)3(s)+ 3HCl  AlCl3(aq) +3H2O(l) How many moles of HCl are required to react with 3.6 moles of Al(OH)3? How many moles of Al(OH)3 will react with 0.042 mol of HCl?

  18. Mg(s) + 2HNO3(aq)  Mg(NO3)2(aq) + H2(g) 0.25mol of Mg is reacted with excess nitric acid How many moles of H2(g) are produced? If one mole of H2 gas has a volume of 24L, what volume of H2 gas is produced? How many moles of nitric acid reacted? If 2 moles of nitric acid were used, how much of the unreacted acid remains?

  19. 40.0g of CuCO3 were heated and decomposed. Ar(Cu) = 63.5; Ar(C) = 12.0; Ar(O)=16.0 Write the balanced equation for the decomposition of CuCO3 to make CuO? Calculate the molar mass of CuCO3 ? Calculate the amount, in moles, of CuCO3 used? Calculate the molar mass of CuO? What is the amount, in moles, of CuO formed? Calculate the mass of CuO formed?

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