Moles, compounds and formula

The formula of a compound can be worked out from the percentage by mass of each element.

Example

Methane contains 75% carbon and 25% hydrogen. What is its Empirical formula?

 

 

C

H

mass (%)

75

25

Moles

75/12

25/1

ratio

6.25

25

 

1

4

Formula = CH4

 

Find the empirical formulae and name each of the following compounds.

 

1)     A black powder is made of 63.2% Mn and 36.8%O.

2)     A dark red powder has the composition Fe - 70%, O - 30%,

3)     A colourless liquid, contains 52.2%C, 13.0%H and 34.8%O

4)     A white powder is Na 27.4%, H 1.2%, C 14.3% and O 57.1 %. When it reacts with an acid it gives out a colourless gas. What is the test for this gas?

5)     An oily liquid that can eat away paper contains 2.1%H, 65.4%O and 32.5%S

6)     A blue crystalline compounds contains Cu 25.0%, S 12.8%, O 25.7% and H2O 36.0%

 

The formulae of compounds can also be determined from experiments involving oxidation or reduction. For each of the following experiments a compound is involved:

I)          Find the formula of the compound

II)         Write an equation for the reaction

 

7)     8.0 g black copper oxide was heated in a tube with a stream of hydrogen passing over it.  6.4g of copper was produced.

 

8)     Two oxides of lead were heated with a stream of hydrogen passing over them. 11.15 g of one oxide produced 10.35 g of lead. 11.95 g of the other oxide also produced 10.35 g of lead.

 

9)     8.96 g of iron was burnt in a flask of chlorine. 26.00 g of a chloride of iron was made.

 

Use the following equations to answer the questions.

 

10)           2Al(s)    +          3FeO(s)            à        Al2O3(s)            +          3Fe(s)

What mass of iron can be produced when 108 g of aluminium is used?

 

11)  Iron is produced in the blast furnace from iron (III) oxide by

Fe2O3(s)              +          3CO(g)                 à        2Fe(s)               +          3CO2(g)

 

What is the maximum mass of iron that can be produced from 357 tonnes of iron(III) oxide.

If the process was 70% efficient what is the mass of iron(III) oxide needed to make 100 tonnes of iron.