Quantitative Chemistry

Question 1

Define Ar.

Answer 1

Relative atomic mass, how heavy atoms are compared to carbon-12 (which is 12).

Question 2

Define Mr.

Answer 2

Relative formula mass, the relative atomic masses added together in a compound.

Question 3

What is the formula for moles?

Answer 3

number of moles = mass in grams/Ar or Mr

Question 4

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

Answer 4

% mass = (Ar x number of atoms (of element))/Mr (of whole compound) x 100

Question 5

What are the five steps to find the empirical formula?

Answer 5

1) list the elements
2) write out experimental masses
3) divide by Ar
4) make whole numbers e.g. x10 then reduce to lowest factors
5) write as the formula

Question 6

How do you calculate mass in reactions? (3 steps)

Answer 6

1) write out balanced equations
2) work out the Ar or Mr (just the bits you want)
3) apply the rule - divide to make 1, multiply to get all

Question 7

What is the formula for percentage yield?

Answer 7

% yield = actual yield (grams)/predicted yield (grams) x 100

Question 8

Why is the percentage yield never 100%? (3 points)

Answer 8

1) reversible reactions
2) unexpected reactions
3) loss when separated

Question 9

Why is product yield important?

Answer 9

For sustainable development e.g. don’t use resources faster than they can be replaced. Also, use as little energy as possible to create the highest yield possible, saving resources. Low yield = wasted chemicals.