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## Relative Masses - Ar and Mr

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### 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. – PowerPoint PPT presentation

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Title: 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

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-1 m(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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