Quantitavtive Chemistry

Question 1

How do you calculate percentage yield? (QC Ch)

Answer 1

The actual yield divided by the theoretical yield (the one worked out in a calculation) times 100.
(A yield x100)
(T yield )

Question 2

How to calculate atom economy? (QC Ch)

Answer 2

The total Mr of desired product
——————————————— x 100 (By Mr it’s the mass, so you use both the big + small no.)
Total Mr of all of the products

Question 3

What are some of the reasons for not 100% yield? (QC Ch)

Answer 3

• reactants left behind in the equipment
• products can get lost when transferring between containers.
• The reactants may be reversible and in these reactions a high yield is never possible as the products are continually turning back into the reactants.
• Some products may be lost during separation and purification stages such as filtration or distillation.
• There may be side reactions occurring where a substance reacts with a gas in the air or an impurity in one of the reactants.

Question 4

Definition of actual yield? (QC Ch)

Answer 4

The recorded amount of product obtained (in an experiment for example)

Question 5

Definition of Theoretical Yield? (QC Ch)

Answer 5

The amount of product that would be obtained under perfect practical and chemical conditions. It is calculated from the balanced equation and the reacting masses.

Question 6

Definition of percentage yield? (QC Ch)

Answer 6

Compares the actual yield to the theoretical yield.

Question 7

Definition of Atom Economy? (QC Ch)

Answer 7

The amount of reactants that get turned into useful products and tells us what percentage of the mass of reactants become useful products.

Question 8

Theory of the Law of Conservation of Mass? (QC Ch)

Answer 8

• Mass is always conserved during a chemical reaction. When a chemical reaction takes place, the relative formula mass of the reactants is always equal to the relative formula mass of the products.
  “No atoms are destroyed or created during a chemical reaction”
• Mass changes occur with changes of state:
  1. Reactants can be atmospheric gases. Some reactants (such as oxygen) can be found in the air. We cannot measure the ‘mass’ of these reactants, since they are free in the air.
  2. Products can be atmospheric gases. Some products (such as carbon dioxide) can be released into the air. We can no longer measure the mass of these products since they are free in the air.