C3- Amount Of Substance And Moles

Question 1

What is meant by amount of substance?

Answer 1

Amount of substance, n, is a quantity used to count the number of particles in a substance.
Measured in mole, mol.

Question 2

What is one mole?

Answer 2

The amount of substance that contains 6.02 x 10*23 particles.

One mole of atoms of any element = its relative atomic mass in grams.

Question 3

What is avogadro constant?

Answer 3

6.02 x 1023 mole-1.

The number of particles in each mole of carbon-12

Question 4

What is molar mass?

Answer 4

The mass per mol of a a substance.

Units= gmol*-1

Question 5

What is the equation for amount of substance?

Answer 5

n = m / M

n= amount, mol
m= mass, g
M= molar mass

Question 6

what is molecular formula?

Answer 6

The number of atoms of each element in a molecule.

Question 7

What is empirical formula?

Answer 7

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

Question 8

What is relative molecular mass, Mr?

Answer 8

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

Question 9

What is relative formula mass?

Answer 9

Compares the mass of a formula unit with the mass of an atom of carbon-12.

Question 10

How do you find the empirical formula from mass?

Answer 10

1. Convert mass into moles using n= m/M
2. Divide both by the smallest number.
3. Write the empirical formula

Question 11

How do you determine the molecular formula?

Answer 11

1. Convert % by mass into moles by using n=m/M
2. Find smallest whole number ratio and empirical formula.
3. Write relative mass of empirical formula.
4. Find number of units in one molecule (Mr / relative mass of empirical formula).
5. Write molecular formula ( empirical formula x ans to part 4).

Question 12

Explain hydrated salts.

Answer 12

Many coloured crystals are hydrated meaning water molecules are part of their crystalline structure.
> this water is known as water of crystallisation. Shown in formula by a ⚫️

Question 13

What happens when blue crystals of hydrated copper (II) sulfate are heated?

Answer 13

• the bonds holding water within crystal= broken and water is driven off.
• leaves behind white anhydrous copper (II) sulfate.

(Anhydrous= contains no water molecules)

Question 14

What are the assumptions made in calculating an experimental formula?

Answer 14

1. All water has been lost:
   
   • you only aww the surface if crystals, some water may be left inside.
   • if hydrated and anhydrous forms are similar colours, it’s hard to tell.
2. No further decomposition:
   
   • many salts decompose even further when heated. Can be difficult to judge if there is no colour change.

Question 15

Equation for calculating amount,n in terms of conc and vol.

Answer 15

n = c x V

n= mol
c= mol dm*-3
V= dm*3 (always convert if given in cm cubed)

Question 16

What is a standard solution and how do you calculate its mass?

Answer 16

A solution of known concentration.

1. Work out amount in moles required (n=cV)
2. Work out molar mass (sum of Mr)
3. Use n=m/M to calculate mass.

Question 17

What is molar gas volume, Vm?

Answer 17

The gas volume per mole at a stated temp and pressure.
Units= dm3 mol-1

At RTP, molar gas volume= 24.0 dm3 mol-1

Question 18

What is RTP?

Answer 18

Room temperature and pressure.

= 20*C and 101kPa (1 atm)

Question 19

What is the equation for amount,n in terms of V and Vm?

Answer 19

n = V / Vm

n=mol
V= dm*3
Vm= 24.0

Question 20

What are the assumptions made for molecules making up an ideal gas?

Answer 20

• random motion
• elastic collisions
• negligible size
• no IMF

Question 21

What is the ideal gas equation?

Answer 21

pV = nRT

p=pressure, Pa
V= volume, m*3
n= amount of gas molecules, mol
R= ideal gas constant= 8.314 mol*-1 K*-1
T= temperature, K

(Conversions on notes)

Question 22

What is stoichiometry?

Answer 22

The ratio of the amount (mol) of each substance in chemical equation

Question 23

What are balanced equations used to find?

Answer 23

• quantities of reactants needed to prepare a required quantity of a product.
• quantities of products that should be formed from certain quantities of reactants.

Question 24

What is theoretical and actual yield?

Answer 24

Theoretical- maximum possible amount of product.

Actual- obtained from reaction (lower than theoretical)

Question 25

Why is theoretical yield hard to achieve?

Answer 25

• reaction may not have gone to completion.
• side reactions may have taken place alongside main reaction.
• purification of product = loss of some product.

Question 26

What is the formula for % yield?

Answer 26

% yield = actual yield / theoretical yield x 100

Question 27

What is limiting reagent?

Answer 27

The reactant that is not in excess, which will be used up and stop the reaction.

Question 28

what is atom economy and its equation?

Answer 28

a measure of how well atoms have been utilised.

Atom economy= sum of molar masses of desired products/ sum of molar masses if all products x 100

> based solely on the balanced chemical equation for a reaction and assumes 100% yield.

Question 29

What does it mean when reactions have high atom economies?

Answer 29

• large proportion of desired products produced, few unwanted waste products.
• important for sustainability as they make the best use of natural resources.

(Some reactions have a much larger atom economy but poor % yield. Efficiency depends on both)

Question 30

What are the benefits of increasing atom economy?

Answer 30

• industrial processes = more efficient
• preserves raw materials.
• reduces waste.

Question 31

How do you calculate relative molecular mass?

Answer 31

Add the relative atomic masses of the elements in the compound.

Question 32

How do you calculate relative formula mass?

Answer 32

Add together the relative atomic masses of the elements in the empirical formula.