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Mole concept

Converting mass to molesProblem Calculate the number of moles of sodium hydroxide formula units present in 1.12g of sodium chloride.

SolutionThe mass of 1 mole of NaOH = 23 + 16 + 1 = 40 gmol-1

40 g of NaOH contain 1 mole of NaOH formula units

1.12g NaOH will contain

= 0.028 mol

11 mol0.028

40.0gmol1.12g

molmol

11

1molmol

1

Formula to convert mass to moles!

# of moles = compound or element per )(gmol mole 1 of mass compound or element of (g) mass given

1-

Converting moles to massProblem What is the mass of 0.75 mole of sulphuric acid?

Solution

Step 1 Write down the formula for sulphuric acid and calculate its formula mass.

H2SO4 = [ ( 2 x 1) + 32 + ( 4 x 16) ] = 98 g mol-1

Step 2 Convert the known number of moles to mass by multiplying by the quantity given; in this case therefore

0.75 mole of sulphuric acid will have a mass equal to

0.75 mol x 98 g per mol = 73.5 g, that is,

73.5g

mol1

98g0.75mol

Important Point

The units are also cancelled in the calculation of the mass.

Formula to convert moles to massTo convert moles to mass:

Mass of a certain number of moles equals the mass of 1 mole multiplied by number of moles, that is,

Mass of a certain number of moles = Mass of 1 mole X Number of moles

Converting moles to number of particlesProblem What is the number of sodium atoms in 0.7 mole of the

element?

Solution

1 mole sodium contains 6.0 x 10 23 sodium atoms

0.7 mole sodium contains

= 4.2 x 10 23 sodium atoms.

0.7mol1mol

atoms sodium 106.0 23

Formula to convert mole to particlesTo convert moles to number of particles:

moles of number number sAvogadro' particles of number

Convert number of particles to molesProblem Calculate the number of moles of aluminium metal which contains 1.2 x 1024 aluminium atoms.

Solution

There are 6.0 x 10 23 aluminium atoms in 1 mole aluminium.

Therefore, 1.2 x 1024 aluminium atoms contain

aluminium moles 2 atoms 106.0

mol 1 atoms 101.2

23

24

Important Point

The units for number of particles, which is actually atoms have cancelled each other. Always pay attention to the units which result so that you know what your answer will be calculated in.

Formula to convert particles to moles

Number of atoms = number of particles number of particles in

one mole the substance

Important Point

The number of particles in 1 mole is the Avogadro number. Avogadro’s number is 6.0 x 1023.

Convert mass to number of particlesTo convert mass to number of particles:

Avogadro’s number x mass (g) divided by mass of 1 mole ( g mol-1)

)(gmol mole 1 of massnumber sAvogadro'

particles of Number1-

Converting Mass to # particlesProblem How many molecules of carbon dioxide are present in

440g of the compound?

Solution

Step 1: convert mass to number of moles( please check original typing expert required)

Number of moles = mass (g)/ mass of 1 mole (g per mole)

Mass of 1 mole carbon dioxide = 12 + ( 2 x 16) = 44g per mole

Number of moles in 440 g of CO2 = 440g/ 44 g mol -1

Worked example cont’d

Formula to convert # particles to massFormula To convert number of particles to mass: number of

particles divided by Avogadro’s number x mass of 1 mole

Problem What is the mass of 3 x 10 exponent 21 molecules of carbon dioxide?

Solution

Step 1: convert number of molecules to moles type the rest of steps from original please!

Step 2: convert moles to mass

( type the rest from original please)

Calculating empirical and molecular formulaeThe empirical formula of a compound

can be readily found from its percentage composition by mass.

The percentage composition by mass of a compound is the percentage by mass of each element in 1 mole of the compound

Empirical formula/ Molecular formulaProblem In an experiment to work out the formula of an oxide of copper the following data were obtained:

Mass of oxide = 3.975 g

Mass of copper = 3.175 g

Mass of oxygen removed = mass of oxide – mass of copper

ie. 3.975 g – 3.175 g = 0.800g.

Solution

Step 1 Calculate the percentage composition by mass of each element using the formula:

Percentage by mass of a particular element in a compound =

100 compound the massof

element particular the of mass total

Worked example cont’d The percentage by mass of a particular element in a compound

= total mass of the particular element divided by the mass of the compound then multiplied by 100. need this to be written

in an equation format. Please type the rest from original writing

Cu O

Percentage composition by mass

79.87% 20.13%

Mass of each element in 100 g of compound

79.87 g 20.13 g

Divide by Ar to get number of moles

79.87 divide by 63.5 = 1.26

20.13 divided by 16 = 1.26

Calculate relative number of moles of each element by dividing by smallest number of moles

1.26 divide by 1.26 = 1 1.26 divide by 1.26 = 1

Therefore the result from this experiment is that :

1 mole of copper atoms combines with 1 mole of oxygen atoms.

Cu O

We then write down the empirical formula of the copper oxide as CuO.

Calculating the Molecular formulaIn order to work out the molecular

formula of a compound you will first need to know the empirical formula. You might have to calculate this from experimental data given.( This is similar to our first example with empirical formula).

We also need to know the mass of 1 mole of the compound. ( Keep Periodic table at hand!)

Computing Molecular formulaeStep 1:Calculate the number of empirical formula

units in 1 mol of a compound by dividing the mass of 1 mol of compound by the mass of 1 empirical formula unit.

( please insert equation from original text)

Computing Molecular formulaeStep 2:Multiply the subscripts of each atom in

the empirical formula by x ( the number of empirical units per mol). This gives the molecular formula of the compound.

Go back to empirical formula problemLet us go back to our example where we

calculated the empirical formula for copper oxide.

- The empirical formula of copper oxide is CuO- The mass of CuO is 79.50g; in other words the relative formula

mass of CuO is 79.50 g- The number of empirical formula units in 1 mol CuO = 79.50g

divided 79.50g = 1

Write down the molecular formula of the copper oxide using your answer. Therefore, Cu O is the molecular formula. However, we write CuO.

Concept map summary of moleconcept