Topic 2.

Question 1

What is the unified atomic mass unit?

Answer 1

One twelfth of the mass of a carbon-12 atom.

Question 2

What is relative isotopic mass?

Answer 2

The mass of an atom of an isotope on a scale compared to the value of the unified atomic mass unit.

Question 3

What is relative molecular mass or relative formula mass (Mr)?

Answer 3

The weighted average mass of a molecule compared to the value of the unified atomic mass unit.

Question 4

What is relative atomic number (Ar)?

Answer 4

The weighted average mass of atoms of an element compared to the value of the unified atomic mass unit.

Question 5

What is Avogadro’s constant (L)?

Answer 5

The number of atoms per mole of a substance.
L = 6.02 x 10^23 mol−1.

Question 6

What is the mole?

Answer 6

The amount of substance which contains
6.02 x 10^23 mol−1 specified particles.

Question 7

What is molar mass?

Answer 7

The mass of a mole of substance in grams.

Question 8

What are the three key equations for calculating moles in:
- Masses.
- Concentration of solutions.
- Gases.

Answer 8

• Masses:
  Number of moles (mol) = mass/ relative molecular/formular mass or Mr.
• Concentration of solutions:
  Number of moles (mol) = concentration (in mol.dm³ or g.dm³) x volume (dm³).
• Gases:
  Number of moles (mol) = volume of gas/molar gas volume (Vm/24.dm³.mol-1).

Question 9

How does one convert:
- cm³ to dm³.
- mol.dm³ to g.dm³.

Answer 9

- cm³ to dm³:
1000cm³ = 1dm³

- mol.dm³ to g.dm³:
to convert from mol/dm³ to g/dm³, multiply by the relative formula mass.
to convert from g/dm³ to mol/dm³, divide by the relative formula mass.

Question 10

What is the formula to calculating concentration?

Answer 10

Concentration = mass/ relative formula mass or Mr x volume.

Question 11

What is empirical formula?

Answer 11

Empirical formula is the simplest whole number ratio of atoms of each element present in a compound.

Question 12

What is the molecular formula?

Answer 12

The actual number of atoms of each element present in a compound.

Question 13

What is water of crystillisation?

Answer 13

Water of crystallisation is water that is chemically bonded into a crystal structure.

Question 14

What is an anhydrous compound?

Answer 14

An anhydrous compound is a compound that does not contain water of crystallisation.

Question 15

What is a hydrated compound?

Answer 15

A hydrated compound is a compound that contains water of crystallisation.

Question 16

What are the two reactions related with anhydrous and hydrated compounds?

Answer 16

• CuSO₄ (white/anyhdrous) ⇌ CuSO₄·5H₂O (blue/hydrated).
• CoCl₂ (blue/anhydrous) ⇌ CoCl₂·6H₂O (pink/hydrated).

Question 17

What is the formula for percentage purity?

Answer 17

Percentage purity = mass of pure substance/mass of impure substance x 100.

Question 18

What is the forumla for percentage yield?

Answer 18

Percentage yield = actual yield/theoretical yield x 100.