3.3 Percentages And Formulae

Question 1

When is finding the percentage of an element in a compound useful?

Answer 1

When deciding if a compound is suitable for a particular purpose or to identify a compound

Question 2

What is the empirical formula?

Answer 2

The simplest ratio of the atoms or ions in a compound

Question 3

Which compounds sometimes have different molecular and empirical formulas?

Answer 3

Covalent compounds

Question 4

Which compounds have the same empirical and molecular formulas?

Answer 4

Ionic compounds

Question 5

Name a covalent compound that has different molecular and empirical formulas.

Answer 5

Ethane

Empirical formula = CH3

Molecular formula = C2H6

Question 6

How can we calculate the empirical formula of a compound when the masses are given?

Answer 6

1. Divide the mass of each element by their Ar’s to give the ratio of atoms
2. If necessary, convert this ratio to the simplest whole number ratio

Question 7

How can we calculate the empirical formula of a compound when the percentage of each element in the compound if given?

Answer 7

1. Divide the mass of element in 100g of compound by the Ar of the element to give the ratio of atoms
2. Convert this into the simplest whole number ratio

Question 8

How do you calculate the percentage of an element in a compound?

Answer 8

(Ar of element / Mr of compound) x100

Question 9

How do we work out the mass of reactants from the formulae of the reaction? (Don’t worry, look at the page in your class book

Answer 9

1. Balance the equation
2. Work out mass in grams of each reactant
3. Divide the mass by the Ar of the element to get the number of moles of that element
4. Work out the ratio of the reactants
5. Divide the number of moles by the ratio of that element to get the percentage
6. Multiply the percentage by the Ar of the other element in the compound and that is the number of grams of the other element in the compound