Chapter 3 - Amount of Substance

Question 1

What is Avogadro’s Constant?

Answer 1

The number of atoms per mole of the carbon-12 isotope.

Question 2

What is a mole?

Answer 2

The amount of any substance containing as many particles as there are particles in 12g of the carbon-12 isotope.

Question 3

What is the equation for the number of moles of a substance?

Answer 3

n=m/Mr
n - number of moles (mol)
m - mass (g)
Mr - molar mass (g/mol)

Question 4

What is molecular formula?

Answer 4

The number of atoms of each element in a molecule.

Question 5

In hydrated salts, what is the name given to water molecules?

Answer 5

Waters of crystallisation

Question 6

What is the concentration of a solute?

Answer 6

The amount of solute (in moles) dissolved in each 1dm^3 of solution

Question 7

What is the equation for finding a solution’s concentration?

Answer 7

c=n/v
c - concentration (mol/dm^3)
n - number of moles (mol)
v - volume (dm^3)

Question 8

What is the value of molar volume at room temperature and pressure?

Answer 8

24 dm^3/mol

Question 9

What are the assumptions made about an ideal gas?

Answer 9

Random motion
Elastic collisions
Negligible size
No intermolecular forces

Question 10

What is the equation for the ideal gas law?

Answer 10

pV=nRT
p - pressure (Pa)
V - volume (m^3)
n - number of moles (mol)
R - ideal gas constant (J/mol/K)
T - temperature (K)

Question 11

What are standard conditions?

Answer 11

298K or 25C

101kPa or 1 atm of pressure

Question 12

What is percentage yield?

Answer 12

The actual yield divided by the theoretical yield, expressed as a percentage.

Question 13

Why is the theoretical yield so hard to achieve?

Answer 13

Reaction is incomplete.
Competing side reactions giving different products.
Loss of product in handling and purification.
Reversible reaction.

Question 14

What is the limiting reagent?

Answer 14

The reactant that is not in excess.

Question 15

What is atom economy?

Answer 15

Molar mass of desired products divided by the sum of the molar masses of all products, expressed as a percentage.

Question 16

What are the benefits of a high atom economy?

Answer 16

Produce a larger proportion of desired products and few waste products.
Important for sustainability, best use of natural resources.

Question 17

For simple molecules, what term would be used

Answer 17

relative molecular mass

Question 18

For compounds with giant structures, what term would be used?

Answer 18

relative formula mass

Question 19

RTP is about

Answer 19

20•C and 101kPa (1atm)

Question 20

1dm^3 =

Answer 20

1000cm^3 = 1000ml = 1 litre

Question 21

What is a standard solution

Answer 21

It is a solution of known concentration

Question 22

How would you carry out an experiment to determine the water of crystallisation

Answer 22

• weigh an empty crucible
• add the hydrated salt into the weighted crucible then weight it
• use a pipe-clay triangle supported on a tripod. Then place the crucible on top.
• Heat the crucible and contents gently for about a minute. Then heat it strongly for a further three miniutes.
• leave the crucible to cool. Then weigh the crucible and anhydrous salt.

Question 23

What assumption do we make when we conduct the hydrated salt experiment and how can you ensure the experiment is accurate as possible?

Answer 23

Assumption 1 - All of the water has been lost. If the hydrated and anhydrous forms have different colours, then it is safe to assume all the water has been lost. If the hydrated and anhydrous forms a similar colours , then it is best to heat to constant mass.

Assumption 2 - no further decomposition
Many salts decompose further when heated; for example, if heated very strongly, copper (II) sulfate decomposes to form black copper (II) oxide. This can be difficult to judge if there is no colour change.

Question 24

Describe the gas syringe experiment to work out the identity of unknown metal

Answer 24

Weigh up a sample of the unknown metal and add to flask

Using a measuring cyclinder weigh an excess of HCL, add 25cm^3 1.0mol dm-3 to the flask and quickly replace the bung connected to a gas syringe

When the syringe has stopped moving, Measure the maximum volume of gas in the syringe

Results: 0.14g of unknown metal
Volume of H2 = 84cm^3

Assuming RTP, n(H2) = V(cm^3) / 24 000

This will give you the moles.

Then you take the moles and use stoichiometry to work out the moles of the unknown metal.

Using that information u can work out the molar mass

Question 25

Efficiency depends on

Answer 25

The atom economy and the percentage yield

Question 26

Improving atom economy =

Answer 26

makes industrial processes more efficient, preserves raw material, and reduces waste

Question 27

Carbon comes from

Steam comes from

Answer 27

Coal

Water

Question 28

{number of moles] =

Answer 28

{number of moles] = mass (g)/ mass of 1 mole (g)

Question 29

How many atoms are there in 11g of propane, C3H8

Answer 29

This is a sneaky question, because it isn’t asking about propane molecules, C3H8, but the total number of atoms. You must read the question very carefully in this sort of problem.

1 mol of propane, C3H8, weighs 44g and contains 6.02 x 10 ^ 23 propane molecules = 1.5 x 10^23

Each propane molecule contains 11 atoms (3 carbons and 8 hydrogen).

Therefore the total number of atoms = 11 x 1.5 x 10 ^23
= 1.65 x 10^24