Mole Ratios in Equations

Question 1

what do the balancing numbers in a balanced equation in a reaction tell us?

Answer 1

• how many moles of each reactant are needed
• how many moles of each product are made

Question 2

if we know the moles of one of the reactants used or products made in a reaction what can we use it for?

Answer 2

we can use the mole ratios in the balance equation to calculate the moles of the other substances

Question 3

example: how would we calculate the mass of N2 needed to react with 1.2g of H2?

Answer 3

1. convert the mass of H2 into moles of H2
2. use the mole ratio from the balancing numbers
3. convert moles of N2 into mass of N2

Question 4

what does it mean if more of less or a substance is needed to complete a reaction

Answer 4

• if there isn’t enough, we say that substance is limiting
• if there is more than enough, we say that substance is in excess

Question 5

example: what happens if we mix 0.9 moles of Zn with 1.1 moles of CuO?

Zn + CuO → ZnO + Cu

Answer 5

the equation tells us the moles of both should be the same so it must be that:

• Zn is limiting, because there isn’t enough of it
• CuO is in excess, because there’s too much of it

Question 6

what does the limiting substance determine?

Answer 6

how many moles are actually used and made

Question 7

how do we determine which is limiting/excess?

Answer 7

• because there are multiple substances in a reaction, each with a mass and potentially different balancing numbers in the reaction equation, it can be difficult to work out which is in excess
• the easiest way is to pick one (at random) then calculate how much of the other would be needed to react with it and then compare that to how much of the other there actually is

Question 8

example: what happens if we mi 10g of Mg and 10g of HCl?

Mg + 2HCl → MgCl2 + H2

Answer 8

• let’s start with Mg, then calculate how much HCl would be needed to react with that amount of
  Mg, and then compare that to how much HCl there actually is
• looks like 10 g of Mg would need to react with 30.4 g of HCl, but we only have 10 g of HCl!
• we now know that HCl is limiting because there isn’t enough, and thus Mg is in excess

Question 9

in an experiment is the mass of the product made as high as it should be?

Answer 9

no

Question 10

how is percentage yield of a reaction calculated?

Answer 10

% yield = experimental / theoretical x100

Question 11

where is the experimental mass measured?

Answer 11

in the experiment - or given to you in the question

Question 12

how is the theoretical mass calculated?

Answer 12

based on the mass of a reactant

Question 13

example: calculate the percentage yield if 10 g of Al produces 0.96 g of H2 in an experiment

Answer 13

1. find or invent a balanced equation for the reaction:
   2Al + 6HCl → 2AlCl3 + 3H2
2. calculated the theoretical mass of H2 that 10g of Al should have produced
3. calculate the percentage yield using the experimental mass from the equation:
   % yield = 0.96g/1.11g x 100 = 86.4%