Past paper questions about number of molecules- Please Help!

There's been 2 questions from past papers which i just really can't get my head around as to why the answers are what they are, and it's one of the first questions on my paper, so supposed to be one of the easiest and worth only a mark each!

So if anyone could explain to me/ show me how you work it out it would be great!! thank you!!


1) State which one of the following contains the greatest number of molecules.
A) 3g of hydrogen
B) 32 g of oxygen
C) 36g of water
D) 66 g of carbon dioxide

2) State which one of the following shows the mass of aluminium that contains the same number of atoms as there are molecules in 11.0g of carbon dioxide, CO2.

A) 6.75g
B) 13.5g
C) 27.0g
D) 54.0g


The answer to 1 is C and 2 is A. But I really don't understand why :??

Molecules have different masses because they are made up of different numbers of atoms.

Divide the mass given by the relative mass of the compound and this gives the number of moles of molecules.

The relative mass of hydrogen (H₂) is 2. Divide 3 by 2 and you get 1.5 moles


Do the same for the other compounds and you get:

B: 32/32 = 1 mole
C: 36/18 = 2 moles
D: 66/44 = 1.5 moles

For the second question the number of moles of carbon dioxide = 11/44 = 0.25.

Do the same calculation for each of the possibilities:

A. 6.25/27
B. 13.5/27
C. 27/27
D. 54/27

Only A = 0.25

Molecules have different masses because they are made up of different numbers of atoms.

Divide the mass given by the relative mass of the compound and this gives the number of moles of molecules.

The relative mass of hydrogen (H₂) is 2. Divide 3 by 2 and you get 1.5 moles

Do the same for the other compounds and you get:

B: 32/32 = 1 mole
C: 36/18 = 2 moles
D: 66/44 = 1.5 moles

For the second question the number of moles of carbon dioxide = 11/44 = 0.25.

Do the same calculation for each of the possibilities:

A. 6.25/27
B. 13.5/27
C. 27/27
D. 54/27

Only A = 0.25

Molecules have different masses because they are made up of different numbers of atoms.

Divide the mass given by the relative mass of the compound and this gives the number of moles of molecules.

The relative mass of hydrogen (H₂) is 2. Divide 3 by 2 and you get 1.5 moles

Do the same for the other compounds and you get:

B: 32/32 = 1 mole
C: 36/18 = 2 moles
D: 66/44 = 1.5 moles

For the second question the number of moles of carbon dioxide = 11/44 = 0.25.

Do the same calculation for each of the possibilities:

A. 6.25/27
B. 13.5/27
C. 27/27
D. 54/27

Only A = 0.25

Mr of hydrogen is 1, not 2

Nope. The Mr of hydrogen refers to the relative molecular mass of hydrogen, i.e the mass per mole of hydrogen - which is a diatomic molecule, i.e. H₂.

The Ar of hydrogen is 1, i.e. the mass per mole of hydrogen atoms.