Measurements, mass and conservation (3.1)

Question 1

What does the law of conservation of mass state?

Answer 1

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

Balance this equation: N₂ + H₂ → NH₃

Answer 2

N₂ + 3H₂ → 2NH₃

Question 3

What is the relative formula mass (Mr) of a compound?

Answer 3

the sum of the relative atomic masses of the atoms

Question 4

In a balanced chemical equation, the sum of the relative formula masses of the reactants (…) the sum of the relative formula masses of the products

Answer 4

equals

Question 5

What is the relative formula mass of CO₂

A_r of Carbon = 12, A_r of oxygen = 16

Answer 5

12 + (2 x 16) = 44

Question 6

Calculate the mass percentage of carbon within CO₂

M_r of CO₂ = 44, A_r of C = 12

Answer 6

12 / 44 = 27.3% (3sf)

Question 7

What is a closed system?

Answer 7

a system in which no substances can enter or leave during a reaction

(mass will remain the same)

Question 8

Why is a bung used in some experiments?

Answer 8

to prevent reactants/products which are gases (or vigorous/spitting liquids) leaving the reaction

Question 9

Why is cotton wool sometimes placed at the neck of the flask?

Answer 9

It allows gas to escape, but nothing else

Question 10

When a metal reacts with oxygen the mass of the oxide produced is (…) than the mass of the metal

Answer 10

greater

Question 11

In thermal decompositions of metal carbonates carbon dioxide is produced and (…) leaving the metal oxide as the (…) solid product

Answer 11

escapes into the atmosphere

only

Question 12

Whenever a measurement is made there is always some uncertainty about the result obtained, so what should we do to gain accurate results?

Answer 12

Repeat the experiments and obtain a mean result