Empirical/ Molecular formulae

Question 1

What is empirical formula?

Answer 1

• Empirical formula is the simplest whole number ratio of
  atoms of each element in a compound.

Question 2

What is molecular formula?

Answer 2

• Molecular formula is the actual number of atoms of each
  element in a compound.

Question 3

What is equivalent to the empirical formula of a compound?

Answer 3

• Molar ratio of compound = empirical formula.

Question 4

Example

Q says: compound contains 30.4% nitrogen/ 69.6% oxygen. Write the empirical formula. What would you do?

Answer 4

• Convert the percentages to masses and work from there (as you are dealing with RATIOS.)

Question 5

How would you work out the molecular formula from a given empirical formula?
Assume you are given the mass of each element in the compound.
You are given the relative molecular mass of the molecular formula.

DO NOT NEED TO LEARN OF BY HEART. IF YOU’RE HAPPY WITH CONCEPT CLICK AND MOVE ON!!

Answer 5

• Calculate the number of moles of each element of the compound.
• Use the moles to calculate the empirical formula.
• Work out the relative molecular mass of compound (from empirical formula.)
• Divide the relative molecular mass by (of empirical formula) by relative molecular mass of molecular formula.
• Multiply the empirical formula (each subsript number) by multiple - calculated-to work out molecular formula.

Question 6

Following fractions as decimals

1/2
1/3
2/3
3/4
1/4
What should you do if you are given a difficult decimal in ratio and you have to make it a whole-number ratio?

Answer 6

0.5
0.3 (recurring)
0.6 (recurring)
0.75
0.25
Convert the difficult decimal into an improper fraction and multiply everything in the ratio by the denominator on improper fraction.