Transcript 887-Chemical Calculations

What does it mean?
11 This is the atomic number :-the no. of protons
Na This is the symbol for the element
23 This is the relative atomic mass
Objectives
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To be able to read data from the periodic
table
To learn how to calculate relative
molecular masses
To understand the significance of
Avogadro’s number and the mole
To carry out calculations on numbers of
particles
Give the Relative Atomic
Masses of the Following
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Sulphur
Chlorine
Barium
Nitrogen
Uranium
Potassium
What does Relative mean?
The mass of an atom is measured relative to 1/12 the mass of a
carbon 12 atom.
It needs to be measured against 1 so why not use hydrogen which
has a mass of 1?
Hydrogen is a reactive colourless gas and is difficult to work with.
Carbon is a stable abundant element and so is used as a comparison
Relative Molecular Mass
This is the masses of all the atoms in a formula added
up.
Eg carbon dioxide has the formula CO2
C has RAM 12
O has RAM 16
So
12 +(2 x 16) = 44
RMM of CO2 = 44
Relative Molecular Mass:Work out the following
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NaCl
MgO
H2SO4
HNO3
Al2(SO4)3
NH4Cl
Activity:- Measure the mass
of a crystal of sugar
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Use the balance to measure the mass of a
single crystal
Can you do it?
Talk to your partner and work out a way to
do it.
How did you get on?
I’m guessing you all weighed out quite a lot of sugar and did some
simple arithmetic????????
Well, atoms are too small to weigh out so chemists use a large
number of of them for convenience
This called AVOGADRO’S NUMBER after the clever bloke who
thought the scheme up.
The NUMBER IS 6 X 10
23
That’s 6 and 23 zeros!
The other name for this number is a MOLE
The Mole
Not a small furry animal:- It’s a really important measurement in
chemistry!
It is 6 x 10
23
particles
It is also the Relative Molecular mass in grams of a compound or
element
Eg. 6 X 10
23
atoms of Mg = 1 mole of magnesium metal atoms
6 x 10
23
molecules of CO2 = 1 mole of carbon dioxide
6 x 10
23
Br- ions = 1 mole of bromide ions
Avogadro’s number
How many particles in 2 moles of particles?
2 x (6 x 10
23)
= 12 x 10
23
How many atoms in 0.5 moles Mg atoms?
0.5 x (6 x 10
23)
= 3 x 10 23
How many atoms in 1 mole of CO2 molecules?
CO2 has 3 atoms in a molecule so 3 x (6 x 10
23)
= 18 x 10
23