49-52 - Calculations

Question 1

What is Ar?

Answer 1

Atomic mass

Question 2

What is Mr?

Answer 2

Relative formula mass

All atomic masses in a compound added together

Question 3

What is the Mr of MgCl2?

Mg - 24, Cl - 35.5

Answer 3

24+35.5+35.5= 95

Question 4

What is a mole equal to?

Answer 4

The Mr of a substance in grams

Question 5

If iron has an Ar of 56, joe much does one mole of iron weigh?

Answer 5

56g

Question 6

Number of moles = ___ / ___ ?

Answer 6

Mass in grams / Mr

Question 7

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

Answer 7

(Ar * number of atoms of element) / Mr of compound

• 100 for %

Question 8

How do you find the empirical formula?

Answer 8

1. Divide the mass (g) of each element by its Ar

2. Multiply and divide all numbers to a simple ratio

Question 9

What mass of MgO is produced when 60g of Mg is burned?

Answer 9

1. Balance the equation (2Mg + O2 -> 2MgO)
2. Work out Mr (224=48 -> 2(24+16)=80)
3. We know 48g -> 80g, so find what 1g gives (80/48 then * 60)
4. 100g

Question 10

How do you find percentage yield?

Answer 10

Actual yield (g) / Predicted yield (g)

• 100 for %

Question 11

What values should percentage yield be between?

Answer 11

0% - 100%

Question 12

What are some reasons for not getting 100% yield?

Answer 12

Reversible reaction

Some product lost in filtering

Unexpected reactions