3- Amount of a Substance

Question 1

What is Avogadro’s Constant (NA)?

Answer 1

The number of atoms in one mole of the carbon-12 isotope. = 6.02 X 10^23

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 relative formula mass?

Answer 4

The weighted mean mass of the formula unit of a compound compared with 1/12th of an atom of carbon-12

Question 5

What is the molecular formula?

Answer 5

The number of atoms of each element in a molecule

Question 6

What is the empirical formula?

Answer 6

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

Question 7

How is the empirical formula found?

Answer 7

1. Mass (%) divided by Molar mass = find the moles of each element
2. Divide all by the smallest answer to find the ratio

Question 8

How is the molecular formula found?

Answer 8

1. Find the empirical formula
2. Divide molar mass of a molecule by molar mass of empirical formula
3. Multiply empirical formula by the result

Question 9

What is relative molecular mass?

Answer 9

The mass of a molecule compared with the mass of an atom of carbon-12

Question 10

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

Answer 10

Waters of crystallisation

Question 11

What is the concentration of a solute?

Answer 11

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

Question 12

What is the equation for finding a solutions concentration?

Answer 12

c=n/v

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

Question 13

What is molar volume?

Answer 13

The volume per mole of a gas (dm^3/mol)

Question 14

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

Answer 14

1 mole of any gas takes up 24 dm^3/mol at rip

Question 15

What is the equation for amount in moles of gas when converting between moles and gas volumes?

Answer 15

n = V/mgv

mgv - molar gas volume (dm^3/mol)
n - number of moles (mol)
V - volume (dm^3)

V (dm^3) - n = V/24
V (m^3) - n = V/24000

Question 16

What are the assumption made about an ideal gas?

Answer 16

Random motion
Elastic collisions
Negligible size
No intermolecular forces

Question 17

What is the equation for ideal gas law?

Answer 17

pV = nRT

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

Question 18

What are the standard conditions?

Answer 18

298 K / 25C
101 kPa

Question 19

What is stoichiometry?

Answer 19

The balancing of numbers giving the ratio of the amount, in moles, of each substance in a balanced equation

Question 20

What are balanced equations used to find?

Answer 20

-the quantities of reactants required to prepare a required quantity of a product
-the quantities of products that should be formed from certain quantities of reactants

Question 21

What is the percentage yield?

Answer 21

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

percentage yield = actual yield / theoretical yield x 100%

Question 22

Why is theoretical yield so hard to achieve?

Answer 22

-the reaction is incomplete
-other reactions (competing side reactions) may have taken place alongside main reaction giving different products
-loss of product in handling and purification
-reversible reaction

Question 23

What is a limiting reagent?

Answer 23

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

Question 24

What is the atom economy?

Answer 24

atom economy = sum of molar masses of desired products / sum of molar masses of all products X100

Question 25

What are the benefits of a high atom economy?

Answer 25

-produce a large proportion of desired products and few waste products
-important for sustainability as they make the best use of natural resources

Question 26

What does the hydrated mean?

Answer 26

Water molecules are part of their (coloured crystals) crystalline structure

Question 27

What does anhydrous mean?

Answer 27

structure that contains no water of crystallisation

Question 28

How do you calculate the formula of a hydrated salt?

Answer 28

1. calculate the amount (in mol) of anhydrous salt
2. calculate the mass and amount in mol of water
3. find the smallest whole-number ratio
4. write down the value of x and the formula of the hydrated salt

Question 29

What is the method for carrying out an experiment to determine the water of crystallisation in hydrated crystals?

Answer 29

1. Weigh an empty crucible
2. Add the hydrated salt into the weighed crucible. Weigh the crucible and hydrated salt
3. Using a pipe-clay triangle, support the crucible containing the hydrated salt on a tripod.
4. Heat the crucible and contents gently for about one minute.
5. Then heat it strongly for a further 3 minutes
6. Leave the crucible to cool, Then weigh the crucible and anhydrous salt

Question 30

Why may the experiment to work out the water of crystallisation not be accurate?

Answer 30

We are assuming that:

-all of the water has been lost (if crystals have changed colour we can assume that all water has been removed but we can only see the surface of crystals and some water could be left inside. If hydrated and anhydrous are similar colours can be difficult to see colour change. SOLUTION = keep heating until the mass of the residue no longer changes)
-there is no further decomposition (many salts decompose further when heated. This can be difficult to judge if there is no colour change)

Question 31

what are the volume measurements commonly used in chemistry?

Answer 31

1 cm^3 = 1ml

1dm^3 = 1000cm^3 = 1000ml = 1 litre

Question 32

What is the value of the ideal gas constant?

Answer 32

8.31 J/mol^-1/K^-1

Question 33

How do you convert from cm^3 to m^3?

Answer 33

X 10^-6

Question 34

How do you convert from dm^3 to m^3?

Answer 34

X 10 ^-3

Question 35

How do you convert from C to K?

Answer 35

+273

Question 36

How do you convert from kPa to Pa?

Answer 36

X 10^3