Unit 3 - Quantitative Chemistry - Law of conservation of mass

Question 1

State 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

Explain what the law of conservation of mass means

Answer 2

This means that chemical reactions can be represented by symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation

Question 3

Mass is always ______ in chemical reactions

Answer 3

Mass is always conserved in chemical reactions

Question 4

What are the reactants

Answer 4

The reactants are the substances we start with before a chemical reaction

Question 5

What are the products

Answer 5

The product is a substance made as a result of a chemical reaction

Question 6

Mg + Cl2 —> MgCl2
24g of magnesium reacted with 71g of chlorine
Calculate the mass of magnesium chloride produced

Answer 6

24 + 71 = 95g

Question 7

MgO + 2HCl –> MgCl2 + H2o
A mass of magnesium oxide reacts with 73g of hydrogen chloride to produce 95g of magnesium chloride and 18g of water
Calculate the mass of magnesium oxide in the reaction

Answer 7

95 + 18 = 113g
113g - 73g = 40g
MgO = 40g

Question 8

State the total number of atoms of Na2CO3 and the number of each atoms of each element

Answer 8

Total number of atoms = 6
Na = 2
C = 1
O = 3

Question 9

Balance the following equation
Fe2O3 + 3CO —> 2Fe + ___CO2

Answer 9

Fe2O3 + 3CO —> 2Fe + 3CO2

Question 10

Define relative formula mass

Answer 10

The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

Question 11

Describe what the sum of the relative formula masses of the reactants in the quantities shown in a balanced equation equals

Answer 11

In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown.

Question 12

Explain why Carbon-12 is given a ‘mass’ of exactly 12 units

Answer 12

Carbon-12 is given a ‘mass’ exactly 12 units because it has six protons and six neutrons

Question 13

Define relative atomic mass

Answer 13

The relative atomic mass of an element is the average mass of the isotopes of that element weighted to take into account the abundance of each isotope

Question 14

State the symbol for Relative Atomic Mass

Answer 14

Ar (subscript r)

Question 15

What atom is the standard atom used that all the other atoms in the periodic table are compared to regarding atomic mass

Answer 15

The standard atom used is Carbon-12 and all the other atoms in the periodic table are compared to this regarding atomic mass

E.g. Hydrogen has a relative atomic mass of 1, as most of its atoms have a mass that is 1/12 of the mass of a Carbon-12 atom.

Oxygen is 16 times bigger than 1/12 or Carbon-12

Question 16

State the equation for the relative atomic mass

Answer 16

Relative atomic mass (Ar) = total mass of all atoms / total number of atoms (total abundance)

total mass of all atoms = (mass number of isotope 1 x percent abundance of isotope 1) + (mass number of isotope 2 x percent abundance of isotope 2)

total abundance/total number of atoms = 100

Question 17

Calculate the relative atomic mass of Chlorine
Isotope - Chlorine-35 and Chlorine-37
Abundance 75.8% and 24.2% respectively
Give your answer to 3s.f.

Answer 17

Ar = (35 x 75.8) + (37 x 24.2) / 75.8 + 24.2

Ar = 3548.4 / 100

Ar = 35.5 (to 3 s.f.)

Question 18

Calculate the relative atomic mass of the element Fluorine
Isotopes = Fluorine-19
Abundance = 100.0%
Give your answer to 3s.f.

Answer 18

Ar = (19 x 100) / 100
Ar = 1900 / 100
Ar = 19.0 (to 3s.f.)

Question 19

State the symbol for Relative Formula Mass

Answer 19

Mr (subscript r)

Question 20

Rules for relative formula mass

Answer 20

Relative formula mass has no units
Relative formula mass never involves big numbers

Question 21

State the relative formula mass of methane

Answer 21

Mr of CH4
12 + (4 x 1) = 16

Question 22

State the relative formula mass of 2CH4

Answer 22

Mr of 2CH4
12 + (4 x 1) = 16

You dont include the big 2.

Question 23

Calculate the Mr of CaSO4

Answer 23

Mr of CaSO4
40 + 32 + (4 x 16) = 136

Question 24

Calculate the Mr of Mg(OH)2

Answer 24

Mr of Mg(OH)2
Mr = 24 + (2 x 16) + (2 x 1) = 58

Question 25

magnesium + sulfuric acid —> magnesium sulfate + hydrogen

Magnesium and sulfuric acid have a combined Mr of 122

Calculate: the total Mr of magnesium sulfate and hydrogen

Answer 25

Mr of MgSO4 and H2 = 122 (because of the law of conservation of mass)

Question 26

What is percentage mass

Answer 26

Percentage mass tells us the percentage of an element in a compound or molecule

Question 27

State the formula percentage mass

Answer 27

Percentage by mass = Mass of element / Mass of compound x 100

Question 28

What percentage of magnesium oxide is actually magnesium

Answer 28

Percentage by mass = 24/(24 + 16) x 100 = 60%

Question 29

Calculate the percentage by mass of calcium in calcium chloride
Mr CaCL2 = 111
Ar Ca = 40

Answer 29

Percentage by mass = Mass of element / Mass of compound x 100

Percentage by mass = 40/111 x 100
Percentage by mass = 36%

Question 30

Calculate the percentage by mass of hydrogen in methane
Mr CH4 = 16
Ar H = 1

Answer 30

Percentage by Mass of H in CH4 = 4 / 16 X 100
= 25%

Question 31

What does 1 mole of any substance contain

Answer 31

1 mole of any substance contains the same number of atoms

Question 32

What is 1 mole

Answer 32

1 mole is the amount of substance in grams which has the same number of particles as there are atoms in 1g of Carbon-12

Question 33

State the unit for moles

Answer 33

Unit for moles = mol

Question 34

What is the mass of one mole of a substance in grams numerically equal to?

Answer 34

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