Moles/ Avagadro's constant/ Limting Reactants

Question 1

EXAMPLE

Calculate the number of C atoms in 11g of carbon dioxide.

HINT HINT, think of it as a decomposition reaction.

Answer 1

1.5 x 10²³(carbon atoms)
write as a decomposition reaction: CO₂ –> C + 2O
- Figure out number of moles of carbon dioxide.
- Go from there! :)

Question 2

How do you workout out mole?

Answer 2

• Moles = mass/ Mr.

Question 3

EXAMPLE

Calculate the number of moles of Al³⁺ ions in 5.1g Al₂O₃?

HINT, HINT, think of it as a decomposition reaction.

Answer 3

• Decomposition: Al₂O₃ –> 2Al³⁺ + 3O²⁻
• 0.05 mol of aluminium oxide.
• (0.05 x 2) = 0.1 mol of Al³⁺

Question 4

Calculate the mass of ONE ATOM of carbon-12

Answer 4

12/ 6.02 x 10²³
= 1.99 x 10⁻²³g

Question 5

Exam Q

Figure out the limiting reactant in this reaction:
CaS + 3CaSO₄ –> 4CaO + 4SO₂
Mass of CaS –> 2.5g
Mass of CaSO4 –> 9.85g

Answer 5

• Moles of CaS (doing 2.5/ Mr) = 0.0346
• Moles of CaSO4 (doing 9.85/ Mr) = 0.0723
• IF IT WAS A PERFECT REACTION, CaS (0.0346 mol) should react with (0.0346x 3) = 0.1038mol but we ONLY have 0.073 mol so, CaSO₄ is the limiting reactant.

Question 6

Mass of sulphur dioxide formed - after having calculated limiting reactant as CaSO4?
CaS + 3CaSO₄ –> 4CaO + 4SO₂
Mass of CaS –> 2.5g
Mass of CaSO4 –> 9.85g

Answer 6

• You have 0.073 mol of CaSO4.
• You use these moles to calculate the mass of SO2 formed (ignore moles of CaS as these are in excess so won’t even react in entirety!)
• 0.073 mol/ 3 x 4 = 0.0973…mol
• 0.09733… mol x 64.1 = 6.2g

Question 7

Standard unit for concentration.

Answer 7

moldm⁻³

Question 8

What is the avagadro’s constant?

Answer 8

• Avagadro’s constant = the number of particles in a mole.