3. Quantitative Chemistry

Question 1

Describe the Law of Conservation of mass

Answer 1

The law of conservation of mass states that no atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants.

Question 2

Why do we balance out chemical reactions?

Answer 2

Chemical reactions are balanced out because of the law of conservation of energy which states that atoms are not made or lost during a chemical reaction, so the number of atoms of the products and reactants must be the same

Question 3

The total mass of reactants is equal to…

Answer 3

The total mass of reactants is equal to the total mass of products

Question 4

What is the relative formula mass?

Answer 4

The relative formula mass (M_r) is the sum of the relative atomic masses of the atoms shown in the formula/compound

Question 5

The total M_r of all the reactant particles is equal to…

Answer 5

The total M_r of all the reactant particles is equal to the total M_r of all the product particles

Question 6

How do we calculate the percentage by mass in a compound?

Answer 6

Question 7

Why do some reactions appear to involve a change in mass?

Answer 7

Some reactions may appear to involve a change in mass because a reactant or product is a gas and its mass has not been taken into account

• For example, oxygen entering an open system and reacting with metals, or gas escaping an open system
• Another example is thermal decomposition, where a metal carbonate breaks down to form metal oxide and carbon dioxide - which escapes into the air

Question 8

What is a closed system?

Answer 8

A closed system is where substances cannot enter or leave

Question 9

What is the issue with taking measurements?

Answer 9

Whenever a measurement is made there is always some uncertainty about the result obtained

Question 10

What is resolution?

Answer 10

Resolution is the smallest change that can be seen or shown

e.g. 1mm in a ruler, 0.1g on a digital balance reading to 1dp

Question 11

How do we measure uncertainities of a single reading?

Answer 11

Uncertainities are given as ± a value
- To calculate the uncertainites when reading a single measurements, we divide the resolution by two and then ± to the reading

Question 12

How do we calculate the range and uncertainity of values in repeat measurements?

Answer 12

In science, the range of a set of values is the difference between the maximum and the minimum value
The uncertainity of a set of values is the range / 2

Question 13

What is the symbol for the unit moles?

Answer 13

Mol

Question 14

What are moles?

Answer 14

Mole is the amount of a substance, where amount refers to the number of atoms it contains

Question 15

Describe the number of atoms, molecules or ions in 1 mole of a substance

Answer 15

• 1 mole of any substance contains the same number of atoms, molecules or ions as one mole of any other substance
• 1 mole of a substance contains 6.02 x 10<sup23</sup> particles

Question 16

What is the mass of one mole of a substance equal to? (e.g. sodium)

Answer 16

The mass of one mole of a substance is numerically equal to its relative formula mass in grams

e.g. the Mr of sodium is 23, so one mole of sodium weighs 23g

Question 17

What is the avogadro constant?

Answer 17

The avogadro’s constant is the number of atoms, molecules or ions in one mole of a substance, which is 6.02 x 10²³

Question 18

Carbon has a relative atomic mass of 12
How many particles are in one mole of carbon?
How much does 1 mole of carbon weight?

Answer 18

• One mole of carbon has 6.02 x 10²³ carbon atoms
• One mole of carbon atoms is 12g

Question 19

FACT, NO ANSWER

In one mole of carbon (C), the number of atoms is the same as the number of molecules in one mole of carbon dioxide (CO2)
(6.02 x 10²³)

Question 20

What is the formula that links moles, relative formula mass and mass together?

Answer 20

Moles = mass / mr

Question 21

Answer 21

3

Question 22

Answer 22

Question 23

What is a limiting reactant?

Answer 23

A limiting reactant is the reactant that is completely used up and limits the amount of products

Question 24

Why is it common to use in excess of one of the reactant in a chemical reaction?

Answer 24

To ensure that all of the other reactant is used

Question 25

How do we find out the limiting reactant?

Answer 25

By calculating the expected and actual moles

Question 26