Topic 3; Quantitive Chemistry

Question 1

What is the law of conservation of mass?

Answer 1

No atoms are lost or made during a chemical reaction- mass of products = mass of reactants

Question 2

Define relative atomic mass

Answer 2

Average mass of atoms in element-taking into account masses and abundance of its isotopes

Question 3

Define relative formula mass

Answer 3

Sum of relative atomic masses of all atoms in the formula

Question 4

What is Avogadro’s constant?

Answer 4

Number of atoms, molecules or ions in a mole of a given substance.

Question 5

What is the formula linking mass, molecular mass and moles?

Answer 5

Mass= moles x Mr

Question 6

How would you calculate the mass of 20 moles of CaCO3

Answer 6

Mass = moles x Mr
Mr = 100
100 x 20 = 2000g

Question 7

Nitrogen and hydrogen form ammonia shown by the following equation:
N2 + 3H2 -> 2NH3
Calculate mass of nitrogen needed to form 6.8 tonnes of ammonia

Answer 7

1. Work out number of ammonia moles
   6800000 / 17 = 400000 moles of ammonia
2. Use this to work out number of moles of nitrogen
   Nitrogen to ammonia is 1:2
   400000/2 = 200000
3. Work out mass of nitrogen
   200000 x 28 = 5600000g = 5.6 tonnes

Question 8

What is a limiting reactant in a chemical reaction?

Answer 8

It is common to use an excess of one reactant to ensure all of the other reactant is used
Reactant completely used up is called limiting reactant as it limits the amount of products

Question 9

Write down 2 formulae linking concentration, mass and volume

Answer 9

Concentration (g/dm) = mass/ volume

Concentration (mol/dm) = no. of moles/ volume

Question 10

What is the molar volume of a gas at room temp?

Answer 10

1 mole of gas occupies 24dm