2.1.3 amount of substance

Question 1

What is a mole?

Answer 1

A unit of measurement to count the number of particles in a substance.

Question 2

What is Avogadro’s constant?

Answer 2

The number of particles per mole, 6.02 x 10^23 mol^-1.

Question 3

What is molar gas and molar gas volume?

Answer 3

Molar gas is the mass per mole, units g mol^-1.
Molar gas volume is the gas volume per mole, units dm^3 mol^-1.

Question 4

What is empirical formula?

Answer 4

The simplest whole number ratio of atoms of each element present in a compound.

Question 5

What is molecular formula?

Answer 5

The number and type of atoms of each element in a molecule.

Question 6

What is the equation for amount of substance?

Answer 6

amount n = mass m / molar mass M

Question 7

How can empirical formula be calculated using mass?

Answer 7

1. convert mass into moles using n=m/M
   2.find the smallest whole number ratio, divide by the smallest whole number
   3.write the empirical formula

Question 8

What is meant by the term anhydrous?

Answer 8

A substance that does not contain water.

Question 9

What is meant by the term hydrated?

Answer 9

When water of crystallisation is present in a crystal compound.

Question 10

What is meant by the term water of crystallisation?

Answer 10

Water molecules that are bonded into a crystalline structure of a compound.

Question 11

What is the calculation of the formula for a hydrated salt?

Answer 11

1.calculate amount of moles of anhydrous CuSO4- n(CuSO4)=m/M
2.calculate the mass and moles of water- n(H2O)=m/M
3.find the smallest whole number ratio- n(CuSO4):n(H2O)

Question 12

What is the practical for a hydrated salt?

Answer 12

1.weigh empty crucible.
2.weigh crucible and salt.
3.heat crucible for one minute gently, then heat strongly for three minutes.
4.leave crucible to cool, then weigh together.

Question 13

What assumptions are made about the practical for a hydrated copper(II) sulphate?

Answer 13

-All the water has been lost.
-No further decomposition.

Question 14

What equation involves concentration and volume?

Answer 14

n = c x V

Question 15

What is a standard solution?

Answer 15

A standard solution is a solution of known concentration.

Question 16

How are standard solutions prepared?

Answer 16

Standard solutions are prepared by dissolving an exact mass of the solute in a solvent and making up the solution to an exact volume.

Question 17

How can mass needed for standard solution be prepared?

Answer 17

1.work out the amount of moles required- n(__)=c x V. (divide volume by 1000 if in cm^3)
2.work out the molar mass
3.rearrange n=m/M to calculate mass required.

Question 18

What equation links molar gas volume and volume?

Answer 18

n = V / Vm

Question 19

What is molar gas volume at room temperature?

Answer 19

Vm=24.0dm^3mol^-1

Question 20

What is the ideal gas equation?

Answer 20

pV=nRT

Question 21

What assumptions are made when using the ideal gas equation?

Answer 21

-random motion
-elastic collisions
-negligible size
-no intermolecular forces

Question 22

What is the ideal gas constant?

Answer 22

8.314 Jmol^-1K^-1

Question 23

What is meant by stoichiometry of reaction?

Answer 23

The balancing numbers give the ratio of the amount of each substance.

Question 24

How can unknown information about a substance be obtained using amounts and an equation?

Answer 24

1.work out the amount in moles of whatever possible.
2.use the equation to work out the amount of moles of unknown chemical.
3.work out the unknown information required.

Question 25

What is the practical for identifying an unknown metal?

Answer 25

1.set up apparatus
2.weigh sample of metal
3.add 25cm^3 1.0 mol dm^-3 HCl (an excess) to flask and replace bung
4.measure maximum volume of gas in the syringe

Question 26

What are the calculations for identifying an unknown metal?

Answer 26

1.n(H2)=vol/24,000
2.using equation X moles can be calculated
3.work out unknown information

Question 27

What is percentage yield?

Answer 27

Percentage yield is the maximum possible amount of product.

Question 28

Why is it difficult to achieve maximum percentage yield?

Answer 28

-the reaction may not have gone to completion.
-other reactions may have taken place alongside the main reaction.
-purification of the product may result in loss of some product.

Question 29

What is the equation for percentage yield?

Answer 29

% yield=actual yield/theoretical yield x 100

Question 30

What is a limiting reagent?

Answer 30

The limiting reagent is the reactant that is not in excess and will be completely used up first and stop the reaction.

Question 31

What is atom economy?

Answer 31

Atom economy is a measure of how well atoms have been utilised.

Question 32

Why are reaction with a high atom economy desired?

Answer 32

-Produce a large proportion of desired products and few unwanted waste products.
-Important for sustainability, they make better use of natural resources.

Question 33

What is the equation for atom economy?

Answer 33

atom economy=sum of molar masses of desired products/sum of molar masses of all products x 100.

Question 34

What are the sustainability benefits of a high atom economy?

Answer 34

-Makes industrial processes more efficient.
-Preserves raw materials.
-Reduces waste.