Quantitative Chemistry

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 (s) + 2 HCl (aq) ➝ MgCl2 (aq) + H2 (g)

Question 3

Define the relative atomic mass and relative formula mass.

Answer 3

• average mass of atoms in an element taking into account masses and abundance of its isotopes, relative to 12C
• sum of RAM’s of all atoms in the formula

Question 4

What is the relative formula mass of:
- CaF2
- C6H12O6

Answer 4

• CaF2 - (Ar values: Ca = 40, F = 19)
  40 + 19 + 19 = 78
• C6H12O6 - (Ar values: C = 12, H = 1, O = 16)
  (12 x 6) + (1 x 12) + (16 x 6) = 180

Question 5

The following reaction occurs in a test tube under a Bunsen Burner:
4 MgO (s) + CH4 (g) → 4 Mg (s) + 2 H2O (g) + CO2 (g)
The carbon dioxide and water escape from the test tube.
Use the equation to explain why.

Answer 5

they are both gases

Question 6

What is Avogadro’s constant?

Answer 6

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

Question 7

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

Answer 7

mass = mr x moles

Question 8

What is the mass of: 20 moles of calcium carbonate, CaCO3

Answer 8

mass = mr x moles
mr = 100
100 x 20 = 2000 g

Question 9

Calculate the amount of carbon dioxide in moles in 0.32 g of carbon dioxide.
Relative atomic masses (Ar): carbon = 12, oxygen = 16.

Answer 9

moles = mass / mr
0.32 / 44 = 0.007

Question 10

Nitrogen and hydrogen form ammonia shown by the following equation:
N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g)
Calculate the mass of nitrogen needed to form 6.8 tonnes of ammonia.
Relative atomic masses (Ar): H = 1; N = 14

Answer 10

Work out the number of number of moles of ammonia (Mr of ammonia = 17)
6800000 / 17 = 400000 moles of ammonia
use a balanced equation and number of moles of ammonia to work out the number of moles of nitrogen
the ratio of nitrogen to ammonia is 1:2
therefore the number of moles of nitrogen is 400000/2 = 200000
work out the mass of nitrogen (Mr of N2 is 28)
200000 x 28 = 5600000 = 5.6 tonnes

Question 11

State what we mean by limiting reactant in a chemical reaction

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 use. The reactant that is completely used up is called the limiting reactant because it limits the amount of products.

Question 12

Hydrogen peroxide decomposes in water to form water and oxygen. How many grams of oxygen gas will be given off from 40.8 g of hydrogen peroxide?