10 minutes maximum! Can you do it in 5?

1. The simplest whole number ratio of atoms in a compound is the .
2. The actual number of atoms in a molecule is the .

3. The empirical formula of the compound formed when 0.2 mole of metal X combines with 0.4 mole of non-metal Y is:

  • B: XY2
  • C: X2Y
  • D: X2Y4
4. The simplest formula for the compound formed when 16g of sulfur reacts completely with 24g of oxygen is:
  • A:  SO       
  • B:  SO2        
  • C:  SO3       
  • D: S2O3
5. The plastic melamine has the empirical formula CH2N2 and a molar mass of 126g/mol. melamine plates

The molecular formula of melamine is:

  • A:  CHN
  • B:  CH2N2
  • C:  C3H6N6
  • D:  C5H10N4


Q6-7. Hydrocarbon X has a relative molecular mass of 42 and contains 85.7% carbon and 14.3% hydrogen.

6. The empirical formula of X is:

  • A: CH
  • B: CH2
  • C: C2H
  • D: C6H1
7. The molecular formula of X is:
  • A:   CH        
  • B:   CH2      
  • C:   C2H4
  • D:   C3H6
8. 0.20g of magnesium oxide was formed by heating 0.12g of magnesium ribbon in a crucible.


heating magnesium in a crucible

Calculate the number of moles of magnesium and oxygen atoms in the magnesium oxide.

  Moles of magnesium atoms Moles of oxygen atoms
A 0.005 0.005
B 0.005 0.0125
C 0.01 0.01
D 0.01 0.025

Q9-10. An oxide of copper was reduced to copper by heating in methane gas in the apparatus shown here:

heating oxide of copper in methane gas

The following results were obtained:

  • Mass of ceramic boat = 20.0g
    Mass of oxide of copper + ceramic boat (before heating) = 24.0g
    Mass of copper + ceramic boat (after heating) = 23.2g

[Relative atomic masses: Cu = 64; O = 16]


9. The masses of copper and oxygen in the original sample are:

  Mass of copper (g) Mass of oxygen (g)
A 4.0 4.0
B 4.0 0.8
C 3.2 4.0
D 3.2 0.8
10. The formula of the oxide of copper is:
  • A: CuO
  • B: CuO4
  • C: CuO5
  • D: Cu5O4