C5 - Calculations ( molar mass, percentage composition, empirical formula)

Question 1

What is Mr and Ar? How do you work out Mr?

Answer 1

relative formula mass = Mr
relative atomic masses = Ar

To work out Mr, add together Ar

Question 2

What is the relative atomic mass of an element? What is molar mass? Units?

Answer 2

• the average mass of an atom of that element, compared with the mass of 1/12th of an atom of carbon-12.
• The mass of 1 mole of a substance (its relative formula mass in grams. ) units = g/mol

Question 3

What is the triangle for moles, mass and molar mass?

Answer 3

Mass

Molar mass Moles

Question 4

Mass is always conserved. Why?

Answer 4

No atoms are created or destroyed during the reaction. They are only rearranged to form new substances.

Question 5

How do you work out how much of a substance you need to make a certain amount of another substance?

Answer 5

RFM ( or mass) of substance you need
——————————————-x the mass you want
RFM ( or mass) of substance you’re going go make

Question 6

What are the steps to find out how much of a substance can be made from a certain amount of another substance?

Answer 6

1. Get the Ar and Mr values

Known mass
————– x Mr of substance you want
Mr you have

Question 7

How do you work out empirical formula ?

Answer 7

1. Write each elements symbol
2. Write each mass or percentage
3. Write each Ar
4. Find moles ( divide 2 by 3)
5. Divide everything by the smallest number
6. Check for whole numbers , write the formula

Question 8

What is empirical formula?

Answer 8

The smallest whole number ratio of each type of atom in a compound .

Question 9

How do I revise this?

Answer 9

Watch videos and answer questions