Module 2: Chapter 3 (Amount of substance)

Question 1

How is mol calculated?

Answer 1

amount, n = mass, m / molar mass, M

Question 2

How is the number of ‘particles’ calculated?

Answer 2

n x Na (6.02 x 10^23)

Question 3

What is the molecular formula?

Answer 3

the number of atoms of each element in a molecule

Question 4

What is the empirical formulae?

Answer 4

the simplest whole-number ration of atoms in each element in a compound

Question 5

What is the water of crystallisation?

Answer 5

• Many crystalline solids are hydrated, water molecules are incorporated into their crystalline structure
• this water is known as the water of crystallisation

Question 6

What is the approach to calculating the formula of a hydrated solid?

Answer 6

• subtract mass of crucible & anhydrous salt from mass of crucible; hydrated salt and use answer to calculate the moles of water
• subtract the mass of crucible from mass of crucible & anhydrous salt and use the answer to calculate the number of moles in the salt
• divide the moles of water by the moles of the compound

Question 7

What is the equation to calculate mol of a gas at RTP?

Answer 7

mol (n) = volume of gas (dm^3) / 24dm^3

Question 8

What is the ideal gas equation?

Answer 8

pV = nRT

p -> pressure (Pa)
V -> volume (m^3)
n -> moles of gas
R -> ideal gas constant (8.314)
T -> temperature (Kelvin)

Question 9

How do you convert between Pa and KPa?

Answer 9

Pa -(÷1000)-> KPa

KPa -(x1000)-> Pa

Question 10

How do you convert between cm^3, dm^3 and m^3?

Answer 10

cm^3 -(÷1000)-> dm^3 -(÷1000)-> m^3
m^3 -(x1000)-> dm^3 -(x1000)0-> cm^3

cm^3 -(÷1 000 000)-> m^3
m^3 -(x1 000 000)-> cm^3

Question 11

How do you convert between K and Celsius?

Answer 11

C -(+273)-> K

K -(-273)-> C

Question 12

What is the concentration of a solution?

Answer 12

The concentration of a solution tells us the amount of solute in a particular volume of solution

Question 13

How is concentration calculated?

Answer 13

concentration (moldm^-3) = mol (n) / volume (dm^3)

Question 14

How do you convert between conc in gdm^-3 and moldm^-3?

Answer 14

gdm^-3 -(÷Mr)-> moldm^-3

moldm^-3 -(xMr)-> gdm^-3

Question 15

What is meant by the ‘amount of substance’?

Answer 15

• n

- counts the number of particles in a substance measured in the unit mole, mol.

Question 16

Explain what a mole is

Answer 16

One mole is the amount of a substance that contains 6.02x10^23 particles (Avogadro constant)

Question 17

What is Avogadro’s constant?

Answer 17

Is 6.02x10^23 mol^-1, the number of particles in each mole of carbon-12
- C-12 is the standard measurement of relative atomic masses

Question 18

What is Molar Mass, M?

Answer 18

• molar mass is the mass per mole of a substance

- the units of molar mass are gmol^-1

Question 19

What is molar gas volume?

Answer 19

• the molar gas volume Vm is the volume per mole of gas molecules at a stated temperature and pressure
• volume of gas depends on the pressure and temperature, most experiments are at RTP

Question 20

What is meant by RTP?

Answer 20

• RTP is about 20 degrees celcius and 101 kPa (1 atm)
• at RTP 1 mole of gas molecules has a volume of approx 24.0 dm^3
• therefore at RTP the molar gas volume = 24.0 dm^3

Question 21

What is meant by the term hydrated?

Answer 21

a crystalline compound containing carbon and hydrogen only

Question 22

What does anhydrous mean?

Answer 22

containing no water molecules

Question 23

How do you calculate percentage yield

Answer 23

actual yield / theoretical yield

x100

Question 24

Why is theoretical yield hard to achieve?

Answer 24

• reaction may not have gone to completion
• other reactions may have taken place alongside the reaction (side reactions)
• purification of the product may result in the loss of some product

Question 25

How is atom economy calculated?

Answer 25

Mr of desired products / Mr of all reactants

x100

Question 26

What are reactions with high atom economies?

Answer 26

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