Percentage composition and empirical formula

Question 1

What is empirical formula?

Answer 1

• Empirical formula is the simplest whole number ratio of each type of atom of a compound.

Question 2

What is the molecular and empirical formula of glucose, methanoic acid, and ethanoic acid?

Answer 2

• Glucose:C₆H₁₂O₆ = CH₂O
• Methanoic acid: HCOOH (or CH₂O₂) = CH₂O₂
• Ethanoic acid: CH₃COOH (or C₂H₄O₂) = C₂H₄O₂

Question 3

Empirical Formula Calculation

Answer 3

1. Write down the mass of each element.
2. Look up the relative atomic mass of each element.
3. Work out how many Moles are of each element (divide by A_r)
4. Choose the element present in the smallest amount.
5. Divide the moles of each of the other elements in the compound by the moles of the one in the smallest amount.

Question 4

Percentage mass Eq.

Answer 4

= total mass of element in the compound (x100)/total mass of compound

Question 6

Calculating empirical formula from percentage composition by mass

Answer 6

1. Mass of each element in 100 grams acid.
2. Relative atomic mass of each element.
3. Convert to moles of each element.
4. Identify the element present in the smallest amount.
5. Divided by the smallest.