Topic 3: Quantitative Chemistry

Question 1

What is 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

Define relative atomic mass

Answer 2

the average mass of atoms in an element taking into account masses
and abundance of its isotopes,

Question 3

what is the sign for relative atomic mass

Answer 3

Ar

Question 4

Define relative formula

mass.

Answer 4

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 5

what is the sign for relative formula mass

Answer 5

Mr

Question 6

key facts about relative formula mass

Answer 6

• has no units

- never involves big numbers

Question 7

why might some reactions appear to show a change in mass?

Answer 7

if the mass appears to change this is because of a gas. if one reactant or product is a gas and if there is a change in the mass this is because the gas has escaped

Question 8

Give two examples of a reaction where a change in mass may appear to take place.

Answer 8

Metal reacting with oxygen or an acid. Thermal decomposition.

Question 9

how would you solve this problem

Answer 9

you could use a sealed container so that it would trap the gas

Question 10

Why does the water, produced during the reaction, have a lower mass than the original hydrogen peroxide?

Answer 10

Because the oxygen gas produced during the reaction escaped into the atmosphere.

Question 11

Chemical measurements

Answer 11

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

Question 12

​The​ ​symbol​ ​for​ ​the​ ​unit​ ​mole

Answer 12

mol

Question 13

number of molecules in one mole =

Answer 13

6.02 x 10 ^23

Question 14

number of moles =

Answer 14

mass (g) / relative atomic mass AR

or mass / relative formula mass

Question 15

mass =

Answer 15

number of moles x Ar
or
number of moles x Mr

Question 16

What is the relative formula?

mass of:

A) CaF2
B) B) C6H12O6

Answer 16

CaF2

• (Ar
  values: Ca = 40, F = 19)

40 + 19 + 19 = 78

C2H12O6

• (Ar
  values: C = 12, H = 1, O = 16)

(12 x 6) + (1 x 12) + (16 x 6) = 180

Question 17

What is Avogadro’s

constant?

Answer 17

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

The value of the constant is 6.02 x 1023

Question 18

What is the formula that
links mass, molecular mass
and moles together

Answer 18

Mass = Mr x Moles

Question 19

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

Answer 19

Mass = Mr x Moles

Mr = 100

100 x 20 = 200 g

Question 20

Calculate the amount of carbon dioxide
in moles in 0.32 g of carbon dioxide.

Relative atomic masses (A
r): carbon =

12, oxygen = 16

Answer 20

Moles = Mass / Mr

0.32 / 44 = 0.007

Question 21

moles=

Answer 21

mass/RAM

Question 22

State what we mean by a

limiting reactant in a
chemical reaction

Answer 22

In a chemical reaction involving two reactants, it is common to use an
excess of one of the reactants to ensure that all of the other reactants is
used. The reactant that is completely used up is called the limiting

reactant because it limits the amount of products.

Question 23

unit of concentration

Answer 23

grams per dm3

g/dm3

Question 24

the equation for concentration =

Answer 24

mass (g) / volume (dm^3)

Question 25

the triangle formula

Answer 25

m ass

c oncentration x v olume

Question 26

higher concentration…

Answer 26

if we keep the volume the same and increase the mass of the solute it gives a higher concentration

Question 27

lower concentration …

Answer 27

if we increase the volume of the solution and keep the mass the same it gives a lower concentration

Question 28

Write down the two formulae that
link concentration, mass and
volume together.

Answer 28

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

Concentration (mol per dm3) = nr of moles/volume (dm3

Question 29

Explain how the mass of a solute and the volume of water effect the concentration of a solution

Answer 29

A larger mass of solute in a certain volume of water → more concentrated solution, [1]
the larger volume of water for a certain mass of solute → less concentrated solution [1]

Question 30

why does the mass decrease when metal reacts with oxygen

Answer 30

The mass of the oxide produced will be larger than the mass of the starting metals because oxygen atoms from the atmosphere have been added

Question 31

what happens in the reaction of Zinc + hydrochloric acid

Answer 31

During this reaction, you would expect the mass of the flask and reactants (zinc and hydrochloric acid) to decrease.
This is because the hydrogen gas produced will escape from the flask.