Chemistry 2 Lesson 3.2 Relative Masses and Moles AQA Additional Science.

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

What have I learnt?

What have I learnt?

What have I learnt?

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

Click for 2 Chemistry Click for 2 Chemistry jokes!!jokes!!

N +

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

What have I learnt?

• 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

The End

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?

Why do cooks use salt?