Quantitive Chemistry

Question 1

What is relative formula mass

Answer 1

All of the relative atomic masses in a compound added together
Mr means relative formula mass

Question 2

Percentage mass of an element in a compound =

Answer 2

Ar X no. of atoms of that element/ X100

Mr of whole compound

Question 3

How to find the empirical formula

Answer 3

List all of the elements in the compound
Write their masses
Divide by their relative atomic masses
Turn the numbers you get into a ratio then simplify

Question 4

How to calculate theoretical yield

Answer 4

Write out the balanced equation
Work out the formula mass of the parts you want
Divide the relative atomic mass to get what you would have with 1g
Then multiply by the actual mass you have

48g of mg gives 80g of MgO
Divide by 48
1g of mg gives 1.67 g of MgO
Times by 60
60g of Mg gives 100g of MgO

Question 5

Percentage yield =

Answer 5

Actual yield (grams)/
Theoretical yield (grams) 

X answer by 100

Question 6

Why are yields always less than 100

Answer 6

Some product or reactant is always lost along the way

Incomplete reactions where not all of the product is converted to the reactants

Practical loses during preparation when the substances are transferred from place to place some is always lost

Unwanted reactions can happen caused by impurities in the reactants or by changing the conditions of the reaction

Question 7

What are the best reaction characteristics

Answer 7

They have a high percentage yield
All products are commercially useful so there’s no waste
The products are made quickly and safely

Disposing of harmful waste products can be expensive