Reacting mass calculations

Question 1

another name for a balanced equation

Answer 1

stoichiometric equation

to do with the ratio of the amounts of reactants and products

Question 2

reactant in excess

Answer 2

if there is enough:

• to react with all the other reactant(s)
• some of it is left over when the reaction ends

Question 3

limiting reactant

Answer 3

other reactant in a reaction with a reactant in excess

not enough to fully react with the other reactant

Question 4

example of an excess/limiting reactant

Answer 4

magnesium + hydrochloric acid:

after reaction:
some magnesium ribbon left

magnesium is in excess
hydrochloric acid is limiting

no magnesium left

magnesium is limiting
hydrochloric acid is in excess

Question 5

limiting reactant and mass of product

how they influence each other

Answer 5

-mass of product formed is controlled by the mass of the reactant not in excess (limiting)

Question 6

length of reaction and limiting reactant

how they influence each other

Answer 6

the reaction continues until all the particles of the limiting reactant have been used up

Question 7

What is the mass of the product directly proportional to?

Answer 7

the mass of the limiting reactant

Question 8

An oxide of copper is heated with excess hydrogen, H2.
In the reaction, 1.27g of copper and 0.18g of water, H2O, form.

Use this information to determine the stoichiometry of the reaction.

(Ar: H = 1, O = 16, Cu = 63.5)

Answer 8

amount of Cu = 1.27/63.5 = 0.02 (in moles/mol - unit for amount of substance, this value is 1 mol)

Mr of H2O = 1+1+16 = 18
amount of H2O = 0.18/18 = 0.01

ratio of Cu:H2O = 0.02:0.01 = 2:1

right hand side must be: -> 2Cu + H2O
so left hand side: Cu2O + H2

equation: Cu2O + H2 -> 2Cu + H2O

Question 9

Explain why, in a chemical reaction, the mass of the reactant not in excess determines the mass of product formed.

Answer 9

reactant not in excess is the limiting reactant

when all its particles have been used up, the reaction stops/no product can be formed

Question 10

Powdered carbon is heated with excess steam, producing 3.3g of carbon dioxide and 0.30g of hydrogen.

Determine the stoichiometry of the reaction.

(Mr: CO2 = 44, H2 = 2)

Answer 10

amount of CO2: 3.3/44 = 0.075 (mol)
amount of H2 = 0.30/2 = 0.015 (mol)
ratio of CO2:H2 = 0.075:0.15 = 7.5:15 = 1:2

right side: -> CO2 + 2H2

equation: C + 2H2O -> CO2 + 2H2