Mass Relationships in Chemical Reactions

1
Mass Relationships in Chemical Reactions

• Chapter 3

2
Empirical Formula of a Hydrate

• Hydrated copper (II) sulfate has the formula
  CuSO4 x H2O.
• To find x, 1.023 g of the blue solid is heated
  in a crucible until its mass no longer decreases.
• The mass of the anhydrous, white CuSO4 is
  0.654 g.
• How many moles of water (x) are there per mole
  of CuSO4?

3
Mass Changes in Chemical Reactions a.k.a.
Stoichiometry
4
Methanol burns in air according to the equation
If 209 g of methanol are used up in the
combustion, what mass of water is produced?
molar mass CH3OH
molar mass H2O
coefficients chemical equation
209 g CH3OH
235 g H2O
5

• Given the following balanced equation,
• 2Mg O2 ? 2MgO
• (a) how many grams of oxygen are required to
  completely react with 0.145 g of magnesium?
• (b) How many grams of magnesium oxide are
  formed?

6
Limiting Reagent

• Reactant that is present in the smaller or
  smallest
• required stoichiometric ratio

2 H2 (g) O2 (g) ? 2 H2O (v)
Fig 3.15
7
2 H2 (g) O2 (g) ? 2 H2O (v)
Fig 3.15

• i.e., in this case O2 is in excess
• and H2 is the limiting reagent (LR)
• The amount of product depends on LR alone!!

8
How do we determine the limiting reagent?
Calculate the theoretical yield twice Assume
first reagent is LR and calculate Assume second
reagent is LR and calculate The smaller result
gives LR and yield
9
Sample exercise 3.19 p 104
How many grams of water will be formed from 150 g
H2 and 1500 g O2 in a fuel cell?
2 H2 (g) O2 (g) ? 2 H2O (v)
Ans 1400 g H2O
This is the theoretical yield
10

• Theoretical yield - the maximum amount of product
  that can be made
• In other words its the amount of product
  possible as calculated through the stoichiometry
  problem.
• This is different from the actual yield, which is
  the amount one actually produces and measures.

11
Sample exercise 3.19 p 104
How many grams of water will be formed from 150 g
H2 and 1500 g O2 in a fuel cell?
2 H2 (g) O2 (g) ? 2 H2O (v)
Ans 1400 g H2O
This is the theoretical yield
Assume, say, 1250 g H2O are formed. Then percent
yield