Relative Formula Mass pg 123

Question 1

How do you find out the Relative formular mass of MgCl2?

Answer 1

Look up the relative atomic masses of all the elements in the compound on the periodic table.

A, of Mg = 24 and the A of Cl = 35.5.

Add up all the relative atomic masses of the atoms in the compound.
Mg = (2 x Cl) = 24 = (2 x 35.5) = 95 So M, of MgCl2 = 95

There are two chlorine atoms in MgCl2, so the relative atomic mass of chlorine needs to be multiplied by 2

(you can find the relative atomic mass (A) of an element from the periodic table - it’s the same as its mass number. See page 97 for more information)

Question 2

What is Relative Formular Mass, M?

Answer 2

The relative formular mass of a substance made up of molecules, is the Sum of the relative atomic masses of the atoms in the numbers shown in the formular.

To find the relative formular mass (Mr) of a compound, you add together the relative atomic mass values (Ar values) for all the atoms in its formular

Question 3

How do you calculate the % Mass of an Element in a compound?

Answer 3

Percentage mass of an Ar x number of atoms of that element
element in a compound = __________________________ x100
Mr of the compound

example:

find the percentage mass of sodium in sodium carbonate, Na2CO3

Ar of sodium = 23, Ar of carbon = 12, Ar of oxygen = 16

Mr of Na2CO3 = (2 x 23) + 12 + (3 x 16) = 106

Percentage mass
of sodium = Ar x number of atoms of that element
_________________________ x 100 = 23x2 x 100 = 43%
Mr of the compound ___
106

Question 4

You might come accross more complicated questions where you need to work out the percentage mass.

A mixture contains 20% iron ions by mass, What mass of ion chloride (FeCl2) would you need to provide the iron ions in 50 g of the mixture?

Ar of Fe = 56, Ar of Cl = 35.5

1) find the mass of iron in the mixture?
2) calculate the percentage mass of iron in iron chloride?
3) calculate the mass of iron chloride that contains of iron?

Answer 4

1) The mixture contains 20% iron by mass, so in 50g there will be 50 x 20
______ = 10g iron
100

2) Percentage Ar x number of atoms of that element 56
mass of iron = ___________________ x 100 = ___________ x 100 = 44.09%
Mr of the compound 56 + (2 x 35.5)

3) Iron Chloride contains 44.09% iron by mass, so there will be

   10g of iron in   ÷  44.09... 
                                 \_\_\_\_\_\_   = 23g
                                   100

So you need 23g of iron chloride to provide the iron in 50g of the mixture.

Question 5

Calculate the relative formular mass (Mr) of:

a) H2O
b) LiOH
c) H2So4

Answer 5

a) Ar of H = 1 and Ar of O = 16
Mr of H2O = (2x1)+16 = 18

b) Ar Li = 7, Ar of O = 16 and Ar of H = 1
so Mr of LiOH = 7 + 16 + 1 = 24

c) Ar of H = 1, Ar of S = 32 and Ar of O = 16
Mr of H2SO4 = (2x1)+32 + (4 x 16) = 98

Question 6

Calculate the percentage composition by mass of potassium in postssium hydroxide (KOH)

Answer 6

Ar of K = 39, Ar of O = 16 and Ar of H = 1
Mr of KOH = 39 + 16 + 1 = 56

39
__ x 100 = 70%
56