Stoichiometry Formulas

Question 1

Given the mass, and the molar mass, how do you calculate the moles of a substance?

• Mass in grams
• Molar mass (Mr or Ar) in g/mol
• Amount of substance, moles in mol

Answer 1

Moles =

mass
_________________
Molar mass

n=m/Mr

Question 2

Given the concentration and the volume, how do you calculate the moles of a substance in an aqueous solution?

• Concentration in mol/dm^3
• Volume in 24dm^3
  (24dm^3 = 24 000cm^3)
• Moles in mol

Answer 2

Moles = Concentration x Volume

n = c x V

Question 3

What is the formula for the calculation of volume in a gas?

1 dm^3 = 1000 cm^3

Answer 3

Volume = Moles x 24dm^3 at r.t.p

v = n x 24dm^3

Question 4

What is the formula for the calculation of percentage yield?

Answer 4

Percentage Yield =
Actual Yield
_________________ x 100
Expected Yield

• Percentage yield is the percentage of a product obtained from a reaction.

Question 5

What is the formula for the calculation of Percentage Purity?

Answer 5

Percentage Purity =
Mass of pure substance
_______________________x100
Mass of sample