Relative Masses - Ar and Mr

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 Ar grams of that element
  there is one mole of atoms
• One Mole 6.02 x 1023
• It is also called Avogadros 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

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Example

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

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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

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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

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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

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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) ?

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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

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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?

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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?

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• 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?