3.1.2 Amount of Substance

Question 1

How many particles in a mole (mol)?

Answer 1

6.02x10(23) (Avogadro’s Constant)

Question 2

What is the equation for Number of particles?

Answer 2

No. of particles = (No. of moles x Avogadro’s Constant)

Question 3

What is the equation to work out the mass of a substance?

Answer 3

Mass = Mr Mol

Question 4

What is the equation to work out the concentration of a solution?

Answer 4

Conc = mol/vol

Question 5

What is the Ideal Gas Equation?

Answer 5

pV = nRT

pressure (Pa) x volume (m3) = no. of mols x 8.31 x Temp (K)

Question 6

How many Pascals (Pa) in a kilopascal (kPa)?

Answer 6

1 kPa = 1000 Pa

Question 7

What do Ionic Equations shows?

Answer 7

Only show the Reacting Particles
e.g.
HNO3 + NaOH ⟶ NaNO3 + H2O would be:
H+ + NO3- + Na+ + OH- ⟶ Na+ + NO3- + H2O
H+ + OH- ⟶ H2O

Question 8

What are the different state symbols?

Answer 8

(s) – solid
(l) – liquid
(g) – gas
(aq) – aqueous

Question 9

What is the equation for number of moles in a solution?

Answer 9

No. of moles, n = (conc, c (mol/dm3) x vol, V (cm3))/1000

Question 10

What is Boyle’s Law?

Answer 10

Product of Pressure and Volume is a constant as long as temperature remains constant

Question 11

What is Boyle’s Law Equation?

Answer 11

Pressure P x Volume V = constant

Question 12

What is Charles’ Law?

Answer 12

Volume is proportional to the temperature as long as the pressure remains constant

Question 13

What is Charles’s Law Equation?

Answer 13

Volume V is Directly Proportional to Temperature T
AND
Volume V ÷ Temperature T = Constant

Question 14

What is Gay-Lussac’s Law (a.k.a. Constant Volume Law)?

Answer 14

Pressure is proportional to the temperature as long as the volume is constant

Question 15

What is Gay-Lussac’s equation?

Answer 15

Pressure P is Directly Proportional to Temperature T
AND
Pressure P ÷ Temperature T = Constant

Question 16

What is the equation if you combine Gay-Lussac’s, Charles’ and Boyle’s Laws equations?

Answer 16

(Pressure P x Volume V) ÷ Temperature T = Constant for a fixed mass of gas

Question 17

How do you make a Standard Solution?

e.g. Make 250cm3 of a 2M solution of NaOH

Answer 17

1) Work out # of moles of NaOH you need using Mol = Conc x Vol – 2 x 0.250 = 0.5 mol
2) Work out mass required using Mass = Mr Mol – 0.500 x 40 = 20.0g of NaOH
3) Place a weighing boat on digital balances and zero the balance, then weigh out 20g of solid. Tip into beaker.
4) Add Distilled (or De-ionised) Water and stir until all NaOH has dissolved
5) Tip solution into Volumetric Flask – make sure it is right size (250cm3 in this case). Use a funnel
6) Rinse the beaker and stirring rod with distilled water and add that to the flask too.
7) Top up flask to correct volume. Make sure bottom of Meniscus reaches the line. When close to the line, add drop by drop – if you go over start again
8) Stopper the flask and turn it upside down/mix a few times

Question 18

How do you do a Titration?

Answer 18

1) Do a rough Titration to get an idea of where the end point is. Add the acid to the alkali using a burette, swirling flask regularly
2) Now do accurate Titration. Take initial reading to see exactly how much acid is in burette. Then run acid in to within 2cm3 of end point. When you reach this point, add dropwise
3) Work out amount of acid used to neutralise the alkali (Titre). This is final reading minus initial reading.
4) Repeat titration a few times, until you have at least 3 concordant results
5) Use results to work out mean Titre - leave out any anomalies

Question 19

What are the two main indicators for Titrations?

Answer 19

1) Methyl Orange - Red in acid, yellow in alkali

2) Phenolphthalein - Colourless in acid, pink in alkali

Question 20

Why should you do Titration of white tile?

Answer 20

Makes it easier to see exactly where end point is

Question 21

What is the Empirical Formula?

Answer 21

Empirical Formula gives simplest whole number ratio of atoms of each element in a compound

Question 22

What is the Molecular Formula?

Answer 22

Gives the Actual Numbers of atoms of each element in a compound
e.g. Glucose = C6H12O6

Question 23

What is Molecular Formula made up of?

Answer 23

Whole number of Empirical Units

Question 24

What are the 3 main steps to work out the Empirical Formula?

Answer 24

1) Find masses of each of the elements present in a compound (by experiment)
2) Work out the number of moles of atoms of each element
No. of moles = Mass of Element ÷ Mass of 1 mol of element
3) Convert the number of moles of each element into a whole number ratio

Question 25

What is the Theoretical Yield?

Answer 25

Mass of the product that should be formed in a chemical reaction. It assumes no chemicals are ‘lost’ in the process

Question 26

What is the equation for Percentage Yield?

Answer 26

Percentage Yield = (Actual Yield ÷ Theoretical Yield) x 100

Question 27

How do you work out the Theoretical Yield of Aspirin?

Mass of Salicylic (2-hydroxybenzenecarboxylic) acid = 6g

Answer 27

Mol of Salicylic = 6 ÷ 138 = 0.0435 mol (to 3sf)
Mr of Aspirin = 180
Mass of Aspirin = 180 x 0.0435 = 7.83g (to 3sf)

Question 28

Example Question
What is the Percentage Yield of Aspirin (2-ethanoylhydroxybenzenecarboxylic acid / Acetylsalicylic Acid) when Theoretical yield = 7.83g and actual yield of Aspirin is 5.95g

Answer 28

5. 95 ÷ 7.83 = 0.7598978289

0. 7598978289 x 100 = 76.0% (to 3sf)

Question 29

What is the equation for Atom Economy?

Answer 29

% Atom Econ = (Mr of Desired Prod ÷ Sum of Mr of all reactants) x 100

Question 30

What is the Atom Economy for Acetylsalicylic Acid (Aspirin) when formed using 2-hydroxybenzenecarboxylic acid (Salicylic Acid) and Ethanoic (or Acetic) Anhydride?

Answer 30

180 ÷ (138 + 102) = 0.75

0.75 x 100 = 75%