4.4.4 - mole ratios in equations

Question 1

the balancing numbers in a balanced equation for a reaction tell us:

Answer 1

• how many moles of each reactant are needed
• how many moles of each product are made
• e.g. 3H2 -> hydrogen has 3 moles

Question 2

method to do it

calculate the mass of N2 needed to react with 1.2g of H2

Answer 2

1. convert the mass of H2 into moles of H2 (moles = mass/RFM)
2. use the mole ratio from the balancing numbers
3. convert moles of N2 into mass of N2

Question 3

what does it mean when a substance is limiting?

Answer 3

there isnt enough

Question 4

what does it mean when a substance is in excess

Answer 4

theres more than enough

Question 5

what does the limiting substance determine?

Answer 5

how many moles are actually used and made

Question 6

whats the easiest way to work out which substance is in excess

Answer 6

• pick one (at random)
• then calculate how much of the other would be needed to react with it
• compare that to how much there actually is

Question 7

how do you calculate percentage yield?

Answer 7

%yield = (experimental/theoretical) x 100

experimental - from experiment, theoretical - from calculation