C2 Topic 6 - Quantative Chemistry

Question 1

What is the Relative Formula Mass of a compound?

Answer 1

The RFM of a compound is the sum of all the RAM’s in the compound.

Question 2

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

Answer 2

Percentage mass of element in compound = ((RAM of element x No. of atoms of that element in compound) / RFM of compound) x 100%

Question 3

How do you calculate the empirical formula of a compound?

Answer 3

1. List all the elements in the compound
2. Write their experimental masses underneath each element
3. Divide each mass by the element’s RAM
4. Turn the numbers into simple integer ratios by multiplying or dividing as necessary
5. This ratio in its simplest form gives the empirical formula of the compound:
   C O
   1 : 2
   CO2

Question 4

How do you calculate the mass of a compound in a reaction?

Answer 4

1. Write out the balanced symbol equation
2. Work out the RFM’s of the compound whose mass is known and that of the one which you are trying to find
3. Divide both RFM’s by the RFM of the compound whose mass is known
4. Multiply both results by the known mass
5. The other result is the mass of the other compound

Question 5

How do you calculate the percentage yield of a reaction?

Answer 5

Percentage yield = (Actual yield / Theoretical yield) x 100%

Question 6

Why are yields almost always less than 100%?

Answer 6

Some product or reactant is lost along the way
Incomplete reactions
Unwanted reactions from impurities
Practical losses due to spillages etc.

Question 7

Why are unwanted products in reactions costly?

Answer 7

They aren’t normally commercially useful

Waste products can be harmful so must be disposed of safely, which costs money

Question 8

What is the Relative Atomic Mass of an element?

Answer 8

The relative atomic mass of an element is the same as its mass number and is equal to the total number of protons and neutrons in the element.