Topic 1 - Calculations involving masses

Question 1

List the limitations of the following models when representing ionic compounds: dot and cross, 2D diagrams and 3D diagrams

Answer 1

• Dot and cross - no lattice structure or ionic bonds.
• 2D diagrams - only shows one layer, doesn’t show formation of ions.
• 3D diagrams - shows spaces between the ions, doesn’t show charges.

Question 2

List the limitations of the following models when representing covalent molecules: dot and cross and ball and stick

Answer 2

• Dot and cross - doesn’t show relative sizes of atoms or intermolecular forces.
• Ball and stick - bonds shown as sticks rather than forces, doesn’t show how covalent bonds form.

Question 3

How do you calculate the relative formula mass of a compound?

Answer 3

Add together all the relative atomic masses of the atoms in the compound.

Question 4

What is the empirical formula? What 2 values could be used to calculate the empirical formula of a simple compound?

Answer 4

• The empirical formula is the smallest whole number ratio of the atoms of each element in a compound.
• Reacting masses or percentage composition can be used to calculate the empirical formula.

Question 5

What is the empirical formula for Fe₂O₄?

Answer 5

FeO₂

Question 6

What is the molecular formula?

Answer 6

Actual number of atoms of each element in a compound.

Question 7

What is the law of conservation of mass?

Answer 7

No matter is lost or gained during a chemical reaction.

Question 8

What equation links mass, moles and relative atomic mass?

Answer 8

Mass (g) = Moles x Relative atomic mass (Mr)

Question 9

How can you calculate concentration in g/dm3 ?

Answer 9

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

Question 10

What is the Avogadro constant?

Answer 10

• The number of atoms, molecules or ions in one mole of a given substance.
• The value of the constant is 6.02 x 1023 .

Question 11

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

Answer 11

Mass (g) = Moles x Relative atomic mass (Mr)

Mr = 100

20 x 100 = 2000 g

Question 12

What formula links the Avogadro constant, moles and number of particles?

Answer 12

Number of particles = Avogadro constant x Moles

Question 13

How many atoms are in 3 moles of copper?

Answer 13

Number of atoms = Avogadro’s constant x Moles

= 6.02 x 10²³ x 3

= 1.81 x 10²⁴

Question 14

What is a limiting reagent in a chemical reaction?

Answer 14

The chemical that is used up first in a reaction, preventing the formation of more product.

Typically, an excess of one of reactants is used to ensure that the other reactant is completely used up. w