Topic 5a: Mass calculations (school curriculum) Topic 1 CPG and PMT

Question 1

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

Answer 1

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

Question 2

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

Answer 2

• 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 3

What is the empirical formula of Fe2O4? (numbers should be little and lower)

Answer 3

FeO2 (2 should be little and lower

Question 4

What is the molecular formula?

Answer 4

Actual number of atoms of each element in a compound.

Question 5

Describe an experiment to work out the empirical formula of magnesium oxide

Answer 5

• Weigh a sample of magnesium
• Heat the sample in a crucible.
• Weigh the mass of magnesium oxide at the end
• Calculate the mass of oxygen (this is the increase in mass).
• Calculate the moles of magnesium and oxygen using the experimental mass and relative atomic mass.
• Work out the whole number ratio of the number of moles of magnesium to oxygen

Question 6

What is the law of conservation of mass?

Answer 6

No matter is lost or gained during a chemical reaction.

Question 7

If a reaction is carried out in a closed system, what can you say about the total mass of the reaction throughout the experiment?

Answer 7

Mass stays constant.

Question 8

If a reaction is carried out in an open flask and a gas is produced, what can you say about the total mass of the reaction throughout the experiment?

Answer 8

Mass decreases as the gas escapes.

Question 9

52g of calcium reacts with oxygen to form 79g of calcium oxide. Using the law of conservation of mass, what mass of oxygen is needed?

Answer 9

79 - 52 = 27

Mass of oxygen = 27g

Question 10

What equation links mass, moles and relative atomic mass?

Answer 10

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

Question 11

How can you calculate concentration in g/dm3?

note: 3 should be higher and smaller

Answer 11

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

note: 3 should be higher and smaller

Question 12

What is Avogadro constant?

Answer 12

The number of atoms, molecules or ions in one mole of a given substance.

The value of the constant is 6.02 x 10 23.

(note: 23 should be higher and smaller)

Question 13

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

note: 3 should be smaller and lower

Answer 13

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

Mr = 100

20 x 100 = 2000 g

Question 14

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

Answer 14

Number of particles = Avogadro constant x Moles

Question 15

How many atoms are in 3 moles of copper?

Answer 15

Number of atoms = Avogadro's constant x Moles
=  6.02 x 10 23 x 3
(note: 23 should be smaller and higher)
= 1.81 x 10 24 
(note: 24 should be smaller and higher)

Question 16

What is a limiting reagent in a chemical reaction?

Answer 16

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

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