5.12 - Molecular formulae

Question 1

What does the molecular formula of a compound show?

Answer 1

The actual numbers of atoms of each element in the compound

Question 2

Can two elements have the same empirical formula, but different molecular formulae?

Answer 2

Yes, because they could be different elements in the same homologous series

Question 3

What pieces of information do you need to calculate the molecular formula of a compound?

Answer 3

• The empirical formula

- The relative molecular mass

Question 4

What is relative molecular mass?

Answer 4

The mass of a molecule, relative to 1/12 the mass of Carbon-12

Question 5

What is a disadvantage of relative molecular mass?

Answer 5

It can only be used with molecules

Question 6

What is an alternative to relative molecular mass?

Answer 6

Relative formula mass

Question 7

What is molar mass?

Answer 7

The mass per mole of any substance

Question 8

What is the equation concerning Molar mass, mass, and moles?

Answer 8

Amount in mol = (Mass of substance in grams)/Molar mass in g mol^-1

n = m/M

Question 9

How do you calculate the molecular formula of a molecule?

Answer 9

• Write down the empirical formula and the relative molecular mass
• Work out the Mr of the empirical formula
• Divide the relative molecular mass by the Mr
• Take the value calculated, and multiply the number of each element in the compound by it

Question 10

What is the ideal gas equation?

Answer 10

pV = nRT

p = Pressure (Pa)
V = Volume (m^3)
n = Number of moles 
R = Gas constant (8.31J mol^-1 K^-1)
T = Temperature (K)