The Mole and the Mass of Substance

8/2/2019 The Mole and the Mass of Substance

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CHAPTER 3:

CHEMICAL FORMULAE &EQUATIONS

C: THE MOLE ANDTHE MASS OF

SUBSTANCES


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You have already learned that 1mole of substance contains

6.02x1023 particles.However it is not practical to countthe number of particles every timewe need to carry out an experiment.

Chemists have a special way of

measuring substance in moles whichis by weighing.

To do so, we need to know the

molar mass of substances


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MOLAR MASS

WHAT IS MOLAR MASS????Molar mass =The mass of one mole ofany substance

=The mass of NA number of

particles=The mass of 6.02x1023

particlesWhat is the unit for molar mass???

Molar mass has the unit of grams per mole orgmol-1


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You have learnt that 1 mole of carbon-12has a mass exactly 12g. Therefore the

molar mass of carbon-12 is 12gmol-1

Molar mass of C-12 = relative atomicmass of C-12 which is 12

molar mass of an element=relativeatomic mass

molar mass of a compound= relativemolecular orformula mass


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ACTIVITIES


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MOLAR MASS AND THE MOLE

By knowing the molar mass of asubstance, we can weigh any fraction ofa mole of the substance

For example: when 12g of C is 1 mol ofC, 6g of C is 0.5 mol and 24g of C is2mol

Molar mass used as the conversationfactor between the number of moles ofany substance and its mass


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mass of substance = number of moles X

molar mass

number of moles = mass/molar mass


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CALCULATION ON THE MOLE ANDTHE MASS OF A SUBSTANCE

1) What is the mass of each ofthefollowing substances???

a) 0.1 mole of magnesium,Mg

b) 2 moles of glucose, C6H12O6

[ relative atomic mass: H=1, C=12,O=16,Mg=24]


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Answera)Molar mass of Mg=24g/mol

Mass of Mg= number of moles X molar mass= 0.1 mol X 24 g/mol= 2.4g

b)Molar mass of C6H12O6= 6(12)+12(1)+6(16)= 180g/mol

Mass of C6H12O6= no. of moles X molar mass= 2mol X 180g/mol= 360g


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2) What is the number of moles ofeach of the followingsubstances???

a) 20g Calcium, Ca

b) 25g Calcium carbonate,CaCO3[ Relative atomic mass:Ca=40, C=12,

O=16]


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3)Calculate the number of

particles ina) 3g of magnesium,Mg

b)4.5g of propanol, C3H7OH[relative atomic mass: Mg=24,C=12, H=1,O=16]


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Answer:

a) 1 mole of Mg= 24g

3g of Mg = 3/24 = 0.125mole

1 mole of Mg contains 6.02x1023atoms.

0.125mole of Mg contains

0.125mole /1 mole X 6.02x1023 atoms.

= 7.5 X 1022 atoms


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b)

1mole of C3H7OH = 3(12)+7(1)+16+1 = 60g

4.5g of C3H7OH = 4.5/60 = 0.075 mole

1 mole of C3

H7

OH contains 6.02x102molecules.

0.075 mole of C3H7OH contains

0.075mole/1 mole X 6.02x10

2

molecules.

= 4.5 X 1022 molecules


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TASK

SOLVE PROBLEMS ON PAGE 35,

WORK THIS OUT 3.2

THE LUCKY STUDENTS WILL BE

SELECTED TO WRITE DOWN THE

ANSWER IN FRONT


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SUMMARIZATION

CLASS, WHAT HAD YOU LEARNT

ABOUT TODAYS LESSON????

The Mole and the Mass of Substance

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