EMPIRICAL AND MOLECULAR FORMULAE

Question 1

What is an empirical formula?

Answer 1

The smallest whole number ratio of atoms of each element in a compound.

Question 2

What are the steps to calculate empirical formulae?

Answer 2

• Divide the mass, or percentage composition by mass, of each element by its relative atomic mass.
• If needed divide the answers from step one by the smallest number.
• This should give an obvious whole number ratio.
• Use them to write empirical formula.

Question 3

Why may the numbers in an empirical formula not be in an exact ratio?

Answer 3

Due to experimental error, you should decide what the nearest ratio is.

Question 4

What is the molecular formula?

Answer 4

Shows the actual number of atoms of each element in a molecule.

Question 5

What is the molar mass?

Answer 5

The mass per mole of a substance.

Question 6

What is the symbol and unit for molar mass?

Answer 6

M and g mol⁻¹

Question 7

How do you calculate mols with molar mass?

Answer 7

mols = mass in grams/molar mass n = m/M

Question 8

How do you work out molecular formula?

Answer 8

Empirical formula, find relative atomic mass of empirical formula. Then divide the molar mass by the formula mass.

Question 9

What is the ideal gas equation?

Answer 9

pV = nRT

Question 10

What are the SI units for the ideal gas equation?

Answer 10

p = pressure in pascals (Pa)
V = Volume in cubic metres (m³)
n = amount of substance in moles (mol)
R = the gas constant (8.31 J mol⁻¹ K⁻¹)
T = temperature in kelvin (K)

Question 11

How do you convert kPa -> Pa?

Answer 11

x10³

Question 12

How do you convert cm -> m?

Answer 12

x10⁻⁶

Question 13

How do you convert dm -> m?

Answer 13

x10⁻³

Question 14

How do you convert °C -> K?

Answer 14

add 273