Post on 03-Jan-2016
Aims
Class work 6 September 2007
Relative Mass and MolesLO’s• What is relative mass?• What’s a mole?
Write it down!! Page 51, 53© C Meyer
All• Know that the masses of atoms can be
compared by using relative atomic mass, Ar
• Work out relative formula mass Mr
• Explain what a mole isMost
• Explain why Ar is different to atomic number
Some
• Explain how Ar relates to isotopes
Aims
Title – Relative Atomic Mass
What is relative mass?
Q10 was - What is the mass number?• Mass number = protons + neutrons
C12
6 If I wanted to react C with O2,could I measure
out the 12 particles
in carbon and 16in oxygen?
Why?m2No, too small to measure!
• We need to be able to measure out amounts to react together, cannot use real masses (too small) so use a relative mass
What is relative mass?
RAM (Ar) – compares the masses of atoms on a scale where carbon is 12 units
Relative atomic mass = 12 (Used as standard)C
m3
• The lightest atom is hydrogen. It has one twelfth the mass of carbon and so has a RAM of 1.
• Magnesium is twice as heavy as carbon. It has a RAM of 24.
Mg
C C
Mg
C C
Mg
C CMg C C
C
H HHH
HH
H HHHH
H
H HHH
HH
H HHHH
H
C
24g x 1 = 12g x 21g x 12 = 12g x 1
What is relative mass?
Mass can be in grams, kg, Te whatever! m3
Relative Atomic Mass
• The Table shows the mass of various atoms relative to carbon.
• Calculate their relative atomic mass.
Element Symbol Times as heavy as carbon R.A.M
Helium He one third
Beryllium Be three quarters
Molybdenum Mo eight
Krypton Kr seven
Oxygen O one and one third
Silver Ag nine
Calcium Ca three and one third
4
9
96
84
16
108
40
Activity
• What is the point of having mass number and relative atomic mass. They both tells us how heavy atoms are!!!
What is relative mass?Element RAM (Ar) Mass Number
H 1 1
He 4 4
C 12 12
What about these elements? What do you notice?
Can you have a mass number that is not a
whole number?You can’t have fractions of neutrons and protonsm2
ISOTOPESISOTOPES
We also have different versions of the same
element’s atoms
WE CALL THEM
ISOTOPESISOTOPESm1
• RAM takes account of isotopes
Isotopes: Chlorine• About 75% of natural chlorine is 35Cl the rest is 37Cl.
Cl35
1775%
17ProtonsElectronsNeutrons
17
18ProtonsElectronsNeutrons
171720
Cl37
1725%
How manyneutrons?
Isotopes of Oxygen
• Almost all of natural oxygen is 16O but about 0.2% is 18O.
• Produce a Table showing the particles in each isotope.
Protons
Electrons
Neutrons
16
O8
Oxygen-18Oxygen-16
8
8
8
Protons
Electrons
Neutrons
8
8
10
Activity
• Isotopes have very similar chemical reactions (same number of protons and electrons.)
• The neutrons only affect physical properties (melting point and density.)
What’s an isotope?
• Draw the electronic structure of carbon 12 and carbon 14
• Say how many protons, neutron• Draw up a table of similar
properties and differences
What have I learnt?
Same Different
What have I learnt?
C126
99% C13
61%
ProtonsElectronsNeutrons
6
6
7
6
6
6
Same Different
Same number electrons Different number neutrons
Same number protons Different mass number
Same chemical properties Different physical props
Same proton number
N
A neutron walks into a bar."I'd like a beer" he says.
The bartender promptly serves up a beer.How much will that be?" asks the neutron.
"For you?" replies the bartender, "no charge."
Two hydrogen atoms walk into a Two hydrogen atoms walk into a bar.bar.
One says, One says, 'I think I've lost an electron.''I think I've lost an electron.'
The other says 'Are you sure?'The other says 'Are you sure?'The first says, 'Yes, The first says, 'Yes, I'm positiveI'm positive... ... ''
+
Title – Relative Formula Mass (Mr)
• If you have the Ar of elements, you can work out the relative mass of any molecule, you just need the formula
What is relative mass?
• Carbon dioxide is ___• How many atoms C = _• How many atoms O = _• RAM of C = __• RAM of = __
CO2
1
2
12
16
CO2 = (1 x 12) + (2 x 16) = 44 = Mr
• Work out the relative formula mass for sodium chloride
What is relative mass?
• Sodium Chloride is ___• How many atoms Na = _• How many atoms Cl = _• RAM of Na = __• RAM of Cl = __
NaCl
1
1
23
35.5
CO2 = (1 x 23) + (1 x 35.5) = 44 = Mr
Ar is relative atomic mass Na23
11
Cl35.5
17
• Work out the relative formula masses ofRAMs Covalent/Ionic
1. Water H = 1, O = 162. H2S H = 1, S = 32
3. Propene C = 12, H = 14. Al2(SO4)3
What have I learnt?
Al27
13S32
16O16
8
1/ H2O = (2 x 1) + (1 x 16) = 18 = Mr
2/ H2S = (2 x 1) + (1 x 32) = 34 = Mr
3/ C3H6 = (3 x 12) + (1 x 6) = 42 = Mr
m8 4/ Al2(SO4)3 = (2 x 27) + (3 x 32) + (12 x 16) = 342 = Mr
Covalent
Covalent
Covalent
Ionic
Title – Moles• Writing relative atomic mass in grams or
relative formula mass in grams is clumsy
• Use a shorthand
What’s a mole?
• A mole is just a number, like a dozen, BUT a lot bigger 6 x 1023
• It’s the number of atoms in the Ar or Mr of a
chemical
m1
• One mole is the RAM or Mr of a substance in grams
What’s a mole?
Mol = Molekulargewicht(German!!)
However, he is Italian
Number of moles = Mass of substance (g) of substance Mass of 1 mole (g/mol)
• Zinc oxide is heated with carbon in a furnace to make zinc and carbon monoxide. How many moles of zinc and carbon are used?
• Write out equation
What’s a mole?
ZnO + C Zn + CO
• One mole is the RAM or Mr of a substance in grams
Mr’s are = 81g + 12g 65g + 18g
• These are all the same number of moles ie 1!!
m5So 1 mole Zn and also 1 mole of C
1. Ca is reacted with chlorine. How many moles of calcium chloride are made?
2. AgNO3 + MgCl2 2AgCl + Mg(NO3)2
How many moles of silver chloride and magnesium nitrate are made?
What have I learnt?
1. Ca + Cl2 CaCl2 1 mol + 1 mol 1 mol
2. AgNO3 + MgCl2 2AgCl + Mg(NO3)2
1 mol + 1 mol 2 mol + 1 mol
m2
What’s a mole?Number of moles = Mass of substance (g) of substance Mass of 1 mole (g/mol)
• Calculate the number of moles of carbon in 36g of the element
Number of moles = Mass of substance (g) = 36g = 3 mol of substance Mass of 1 mole (g/mol) 12g/mol
• Calculate the number of moles of carbon dioxide in 33g of the gas
Number of moles = Mass of substance (g) = 33g = 0.75mol of substance Mass of 1 mole (g/mol) 44g/mol
O16
8C12
6m4
• Lets look at Chlorine. It exists as 2 isotopes
• Isotopes• Write down to number of
protons, neutrons and electrons for the 2 isotopes of chlorine
3517 Cl and 37
17 Cl
What is relative mass?
• 3717 Cl has 2 more neutrons
• Cl gas is 75% Cl-35 and 25% Cl-37• So in 100 atoms of Cl, 75 are Cl-35 and 25 are Cl-37• Work out the mass in 100 atoms (relate to H)
m1
• (75atoms x 35g) + (25atoms x 37g) = 3550g• So the average mass is 3550 divide 100 =
35.5g
What is relative mass?
m2
• RAM takes account of isotopes
Quiz, In front1. Define a compound2. Give an example of an ionic bonded compound3. Give 2 properties of ionic compounds4. Using 1 of the properties above, explain why
their structure gives an ionic compound that property
5. What is a covalent bond?6. Give an example of a covalently bonded material7. Give 1 property of a covalent material8. Why can graphite be used as a lubricant?9. Carbon has a mass number of 12 and atomic
number of 6. Write this is short hand10. What is the mass number?
What have I learnt?
over
• Are solid at room temp• Have high melting points• Soluble in water• Conduct heat• Conduct electricity when in
solution or molten
Particles closeStrong ionic bonds
Ionic bonds
Vibration and delocalised electrons
Charged ionsfree to move
What have I learnt?