3.1 Quantitative chemistry (Triple)

Question 1

What is the law of
conservation of mass?

Answer 1

The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.

Question 2

Write a balanced equation
of magnesium reacting with
hydrochloric acid.

Answer 2

Mg + 2 HCl → MgCl2 + H2

Question 3

Define relative atomic mass
and relative formula mass.

Answer 3

Relative atomic mass - average mass of atoms in an element taking into account masses and abundance of its isotopes, relative to Carbon-12.

Relative formula mass - sum of the relative atomic mass of all atoms in the formula.

Question 4

What is the relative formula
mass of: CaF2

Ca = 40
F = 19

Answer 4

78

Question 5

What is the relative formula
mass of: C6H12O6

C = 12
H = 1
O = 16

Answer 5

180

Question 6

The following reaction occurs in a test tube under a Bunsen
Burner:

4 MgO(s) + CH4(g) → 4 Mg(s) + 2H2O(g) + CO2(g)

The carbon dioxide and water escape from the test tube.
Use the equation to explain why.

Answer 6

They are both gases

Question 7

Suggest
how you could increase the precision
of results.

Answer 7

Measure to more decimal places or use a more sensitive balance /
apparatus

Question 8

What is Avogadro’s
constant? (Higher)

Answer 8

The number of atoms, molecules or ions in a mole of a given substance.
The value of the constant is 6.02 x 10^23

Question 9

What is the formula that
links mass, molecular mass
and moles together? (Higher)

Answer 9

Mass = Mr x Moles

Question 10

What is the mass of 20 moles of calcium carbonate, CaCO3 (Mr is 100)?

Answer 10

2000g

Question 11

State what is meant by a limiting reactant in a chemical reaction. (Higher)

Answer 11

In a chemical reaction involving two reactants, it is common to use an
excess of one of the reactants to ensure that all of the other reactants are used. The reactant that is completely used up is called the limiting
reactant because it limits the amount of products.

Question 12

Write down the two formulae that
link concentration, mass and volume together.

Answer 12

Concentration (g per dm3) = Mass (g)/Volume (dm3)

Concentration (mol per dm3) = no of moles/volume (dm3)

Question 13

What is the molar volume of a gas
at room temperature and
pressure?

Answer 13

1 mole of a gas at room temperature and pressure occupies 24 dm3

Question 14

What is titration?

Answer 14

A technique for finding the concentration of a solution by reacting a known volume of this solution with a solution of known concentration.

Question 15

How do you conduct a titration?

Answer 15

1. Rinse the pipette with a solution of unknown concentration. Use the pipette to
   measure out the known volume of this solution.
2. Add an indicator (a substance that changes colour at the end of titration)
3. Rinse the burette with a solution of known concentration. Discard the liquid.
   Use a burette to gradually add the solution of a known concentration.
4. When the indicator changes colour (at the end point), the volume added is
   recorded
5. It is important to get concordant volume results - they have to lie close to each
   other
6. Suitable calculations are performed to find the concentration.

Question 16

Why is it not always possible to
obtain the theoretical amount of product in a chemical reaction?

Answer 16

● The reaction may not go to completion because it is reversible.

● Some of the product may be lost when it is separated from the
reaction mixture.

● Some of the reactants may react in ways different to the expected reaction (side reactions may occur).

Question 17

How is the percentage yield of
a product calculated in a chemical
reaction?

Answer 17

% Yield =
Actual mass of a product / Maximum theoretical mass of product
x100

Question 18

What is atom economy?

Answer 18

A measure of the amount of starting materials that end up as useful products. It is a ratio of the relative formula mass of the desired product to the sum of relative formula masses of reactants.