SC9: Calculations Involving Masses

Question 1

SC9a Masses and empirical formulae

A) What is an empircal formula?
B) Find the empircal formula of;
- 1. C2H6, 2. N2H4 3. C3H8, 4. C6H6

Answer 1

A) This is the simplest whole number ratio of atoms or ions of each element in a substance.

B1) Answers are;
1. CH3, 2. NH2, 3. C3H8, 4. CH

Question 2

SC9a Masses and empirical formulae

How would you find the molecular formula?

Answer 2

1. Empirical formula mass / relative atomic mass = X

2. Multiply X by the empirical formula.

Question 3

SC9a Masses and empirical formulae

Describe an experiment to determine the empirical formula of magnesium oxide.

Answer 3

1. First, heat magnesium ribbon in a limited oxygen supply.

2. If the reactant and product are ‘weighed’ the empirical formula can then be calculated.

Question 4

SC9b Conservation of mass

A) What is the law of conservation of mass?
B) Explain it in relations to a closed system.
C) Explain it in relations to a non-enclosed system.

Answer 4

A) When a solute is dissolved in a solvent to make a solution, the mass before the reactions and after will be the same.
- This is the law of conservation of mass.

B) Lead nitrate solution reacts with potassium iodine solution to form a yellow precipitate, of lead iodide and a colourless solution of potassium nitrate.
- This is a closed system as no new substances are added or removed

C) However, in a reaction involving a gas, like Carbon dioxide, the mass may decrease due to some of the gas escaping.
- This is a non-enclosed system.

Question 5

SC9b Conservation of mass

What is the formula for concentration of a solution in g dm-3?

Answer 5

Concentration = Mass of solute in g/Volume of solution in dm3

1 dm3 = 1 Litre or 1000cm3

Question 6

SC9c Moles

What is a ‘Mole’?

Answer 6

• A mole is the amount of particles within a certian substance.
• This is also referred to as the Avogardo constant.

Question 7

SC9c Moles

Describe what is ment by a ‘limiting reactant’.

Answer 7

• This refers to how in a reaction one of the reactant is usually added in excess to ensure the a full reaction takes place.
• So when calculating the amount of product formed, we look at the amount of reactant that was completely used up.
• This is called the limiting reactant.

Question 8

SC9c Moles

What is ment by the stoiciometry of a reaction?

Answer 8

The ratio of moles in each substance.