C2b Topic 6 - Quantitive Chemistry

Question 1

The Relative Formula Mass, Mr, is just

Answer 1

All the relative atomic masses added together!

Question 2

Percentage mass of AN ELEMENT IN A COMPOUND =

Answer 2

1) The Ar (relative atomic mass) TIMES BY no. of atoms (of that element)
2) DIVIDE THAT ^ by the Mr (relative formula mass) of the whole compound
3) TIMES BY 100

Question 3

How to find the Empirical Formula (from masses)

Answer 3

1) List all the elements in the compound.
2) Underneath them, write their experimental masses.
3) Divide each mass by the Ar (relative atomic mass) for that particular element.
4) Turn the numbers you get into a nice simple ratio by multiplying and/or dividing them by well-chosen numbers.
5) When the ratio is in its simplest form, this tells you the empirical formula of the compound.

Question 4

Three important steps for calculating masses in reactions

Answer 4

1) Write out the balanced equation.
2) Work out the Mr (relative formula mass) - just for the two bits you wanted.
3) Apply the rule : divide to get one, then multiply to get all. (But you have to apply this first to the substance they give information about, and then the other one!)

Question 5

Percentage yield =

Answer 5

Actual yield in grams DIVIDED BY theoretical yield in grams then TIMES BY 100

Question 6

Yields are always

Answer 6

Less than 100% :(

Question 7

Three reasons why yields are always below 100%

Answer 7

1) Incomplete reactions - if not all of the reactants are converted into product, the reaction is incomplete and the yield will be lower than expected.
2) Practical losses during preparation - you always lose a bit when you transfer chemicals from one container to another. Think about transferring a liquid to a new container - some of it always gets left behind on the inside surface of the old container.
3) Unwanted reactions - things don’t always go to plan. Sometimes you get unexpected reactions happening, so the yield of the intended product goes down. This can be caused by impurities in the reactants, but sometimes just changing the reaction conditions affects what products you make.

Question 8

Waste in reactions…

Answer 8

Costs money

Question 9

If a waste product is harmful, it can pose a threat to people and the environment. Disposing of harmful waste products safely can be

Answer 9

Very expensive

Question 10

The Relative Atomic Mass, Ar, for each element is the same as

Answer 10

It’s mass number!