2.1.3 Amount Of Substance

Question 1

What is a mole?

Answer 1

An amount of substance containing the same amount of particles of 12g of a carbon-12 isotope.

Question 2

How do you find amount of substance?

Answer 2

Mass (g) / Molar mass

Question 3

What is relative molecular mass?

Answer 3

The mass of a molecule relative to 1/12th of the mass of a carbon- 12 atom.

Question 4

What is relative formula mass?

Answer 4

The mass of a formula unit relative to 1/12th of the mass of a carbon-12 atom.

Question 5

How do you find concentration?

Answer 5

Concentration = amount/volume

Question 6

What is Avogrado’s constant?

Answer 6

The amount of particles in one mole of a substance. (Remember to use specific terms.)
6.02 × 10²³

Question 7

What is Avogrado’s constant?

Answer 7

The amount of particles in one mole of a substance. (Remember to use specific terms.)
6.02 × 10²³

Question 8

What are room temperature conditions?

Answer 8

298K and 101000Pa

Question 9

How to find volume of a gas at RTP?

Answer 9

Volume = 24dm³ × amount (moles)

Question 10

How to find the volume of gas not at RTP?

Answer 10

pV= nRT

Pa × m³ = mol × 8.31 × K

Question 11

How do you convert from dm³ to m³

Answer 11

(×10-¹)³
× 10-⁴

Question 12

How do you convert from degrees Celsius to kelvin?

Answer 12

+ 273

Question 13

What is molar gas volume?

Answer 13

Volume per mole of gas molecules at a stated temperature and pressure.

Question 14

What assumptions do we make when using the ideal gas equation?

Answer 14

Negligible size.
No intermolecular forces.
Random motion.
Elastic collision.

Question 15

Why do chemist’s use balanced equations?

Answer 15

To find :
The quantities of reactants required to prepare a required amount of product.
The quantity of product produced through a fixed quantity of reactants.

Question 16

What is the limiting reagent?

Answer 16

The reagent not in excess.

Question 17

What is atom economy?

Answer 17

The measure of how well atoms are utilised .

Question 18

How do we work out atom economy?

Answer 18

(Mr of desired product/ Mr of all products ) ×100%

Question 19

How does high atom economy help?

Answer 19

Produce high proportions of desired product.
Decreases waste.
Increases sustainability.
Better use of natural resources.

Question 20

What is percentage yield?

Answer 20

How much product is obtained compared to the maximum possible mass.

Question 21

How do we calculate percentage yield?

Answer 21

(Mr of actual yield/Mr of theoretical yield) × 100%

Question 22

How do we calculate percentage yield?

Answer 22

(Mr of actual yield/Mr of theoretical yield) × 100%

Question 23

What might cause percentage yield to be less than a 100%?

Answer 23

The reaction may not have gone into completion.
Side reactions may have taken place, creating different products.
Products may be lost as a result of transfer or purification.
Unexpected reactions have taken place , causing loss of product such as gases.

Question 24

What is water of crystallisation?

Answer 24

Water molecules that are chemically bonded into a crystalline structure.

Question 25

What happens when hydrated salts are heated?

Answer 25

Bonds holding the water are overcome and the water is driven off, leaving an anydrous white powder.

Question 26

How do we find water of crystallisation in practicals?

Answer 26

1) Weigh an empty crucible
2) Add hydrated salt and reweigh, on a top pan balance.
3) Use a pipe clay triangle to support the crucible, and heat gently for 1 minute, and strongly for 3 minutes.
4) Please crucible to cool and reweigh.
5) Calculate the moles of anhydrous salt .
Calculate moles of water.
Find the smallest number ratio, by dividing by the smallest, and find x.
(E.g CuSO4 • xH2O)

Question 27

What assumptions do we make in the water of crystallisation practical how do we correct them?

Answer 27

All water has been driven off - heat and reweigh continually
No further decomposition has taken place - look for colour change
(CuSO4 turns black , metal carbonates might produce metal oxides)

Question 28

How do we find the relative molecular mass of a volatile liquid?

Answer 28

1) Add a sample of volatile to a small syringe via a needle.
2) Weigh the syringe on a top pan balance
3) Inject sample into a gas syringe through the self sealing rubber cap and reweight the small syringe, to find the mass of the volatile liquid added to the gas syringe.
4) Place the gas syringe in water until the sample vaporises , forming a gas.
5) Use the mass of liquid, volume of gas , atmospheric pressure and temperature to find moles . We can use this to find the molecular mass.

Question 29

How do you prepare a standard solution?

Answer 29

1) Weigh solid accurately.
2) Solid should be dissolved in distilled water, i a beaker , using less distilled water than is needed to fill the volumetric flask.
3) Transfer the solution to the volumetric flask, rinsing the last traces from the beaker using distilled water. 4) Carefully fill the volumetric glass to the graduation line with distilled water, making sure the bottom of the meniscus rests on the line at eye level. 5) Invert the flask slowly. (If this stage is omitted, that titration is likely to be inaccurate.)

Question 30

How to simply do a titration?

Answer 30

1)Rinse a pippette with distilled water, then alkali and use it to measure out a known volume of alkaline ( slowly to avoid bubbles .)
2) Rimseconical flask with distilled water and transfer the alkali to the conical flask .
3) Add a small amount of indicator indicator to conical flask.
4) Rinse a burettewith distilled water and then acid and clamp at eye level.
5) Fill the burette with acid using a funnel and remove the funnel .
6) Place the alkali on a white tile, and open the tap and swirl the conical flask until a colour change takes place.

Question 31

What precautions should I use when doing a titration?

Answer 31

Rinse pippette with water then alkali
Use the pipette slowly to avoid bubbles.
Make sure the end of the pippette is always under the water as the water level falls.
Make sure the bottom of the meniscus always lines up with the graduation.
Place the tip of the pipette into the water to dispel the last dregs.
Rinse the burette with distilled water .
Remove the funnel to prevent dripping.
Place white Paper behind the burette to make the units more clear.
Preform titrations until two or more concordant results are achieved.

Question 32

What indicator do we use in the titration of a strong acid and strong alkali?

Answer 32

Phenolpthelein or methyl orange

Question 33

What indicator do we use in the titration of a weak acid?

Answer 33

Pthenolpthalein

Question 34

What indicator do we use in the titration of a strong acid?

Answer 34

Methyl orange

Question 35

What can I titration calculations be used for?

Answer 35

Finding concentration.
Identification of unknown chemicals.
Finding purity of a substance. (Important for quality control)
Finding water of crystallisation .