TOPIC 1: MOLE CONCEPT & STOICHIOMETRY

Question 1

[Definition] Isotopes

Answer 1

Isotopes are atoms of the same element whose nuclei have the same number of protons but different number of neutrons.

Question 2

[Definition] Relative Isotopic Mass

Answer 2

Relative isotopic mass is the ratio of the mass of an atom of isotope to 1/12 of the mass of a Carbon-12 atom.

*No units

Question 3

[Definition] Relative Atomic Mass

Answer 3

Relative atomic mass is the ratio of the weighted average mass of an atom of an element to 1/12 of the mass of a Carbon-12 atom.

*No units

Question 4

[Definition] Relative Molecular Mass

Answer 4

Relative molecular mass is the ratio of the weighted average mass of a molecule to 1/12 of the mass of a Carbon-12 atom.

*No units

Question 5

[Definition] Relative Formula Mass

Answer 5

Relative formula mass is the ratio of the weighted average mass of a formula unit of an ionic compound to 1/12 of the mass of a Carbon-12 atom.

*No units

Question 6

[Definition] Mole

Answer 6

One mole of any substance is the amount of that substance which contains 6.02 x 10^23 elementary entities.

*Elementary entities must be specified (can refer to atoms, molecules, ions, or electrons)

Question 7

[Formula] 4 Main Formula

Answer 7

no. of particles = amt (in mol) x 6.02 x 10^23

Mass = amt (in mol) x molar mass

Volume = amt (in mol) x molar volume

Concentration = amt (in mol) / volume (in dm3)

Question 8

[Conversion] Volumes

Answer 8

1 dm3 = 1000 cm3 = 0.001m3 = 1000ml = 1litre

Question 9

[Definition] Empirical Formula

Answer 9

Empirical formula is the formula that shows the simplest whole number ratio of the number of atoms of each element present in the substance.

Question 10

[Definition] Molecular Formula

Answer 10

Molecular formula is the formula that shows the actual number of atoms of each element in one molecule of the compound.

Question 11

[SOP] Finding empirical / molecular formula

Answer 11

Mass / g
Ar
Amount / mol
Simplest mole ratio

Let molecular formular of X be C3nH4nO5n.
Mr of X = n ( 3 x … + 4 x … + 5 x …) = 114
So, n = 2
So, molecular formula of X is …

Question 12

[Formula] Percentage Yield

Answer 12

% Yield = actual mass / theoretical mass x 100%

% Yield = actual amt (in mol) / theoretical amt (in mol) x 100%

Question 13

[SOP] Solving combustion reaction questions

Answer 13

1. Write out formula - CxHy + (x+y/4)O2 –> xCO2 + (y/2)H2O
2. Based on volume of CxHy, find stoichiometric ratios of the rest
3. Solve based on volumes provided by question

Question 14

Difference between equivalence point and end point

Answer 14

Equivalence point: Point when reactants just reached complete reaction according to stoichiometric ratio

End point: Point where indicator changed its colour to show complete reaction

In short,
equivalence point = theoretical completion of reaction
end point = practical completion of reaction

Question 15

What’s the difference between a strong and weak acid/base reaction in titration?

Answer 15

The ending pH of the solution

BOTH strong and weak acid/base WILL REACT COMPLETELY in titration reactions !!!

Question 16

[Formula] Dilution of solutions

Answer 16

c1V1 = c2V2

*Concept: amt of substance (mol) is the same before and after dilution

Question 17

[Definition] Redox Reaction

Answer 17

A reaction in which oxidation and reduction take place at the same time

Question 18

Oxidation VS Reduction

Answer 18

Gain of oxygen, Loss of hydrogen,
Loss of electrons, Gain in oxidation number

Loss of oxygen, Gain of hydrogen,
Gain of electrons, Loss in oxidation number

Question 19

MnO4- in acidic and neutral/alkaline medium

Answer 19

[Acidic] MnO4- to Mn2+ (pale pink / colourless if in dilute solution)
[Neutral/alkaline] MnO4- to MnO2 (brown solid)

Question 20

[SOP] Order of assigning oxidation numbers (standard)

Answer 20

Group 1 (always +1)
Group 2 (always +2)
Aluminium (+3)
F (-1)
O (-2)
H (+1)

Question 21

[SOP] Order of assigning oxidation numbers (complicated molecules)

Answer 21

1. More electronegative atom gets the negative sign, the other gets the positive sign
2. Single bond, +/-1 ; Double bond, +/-2 ‘ Trible bond, +/-3
3. Average oxidation number = sum of oxidation numbers / number of atoms

Question 22

[Definition] Electronegativity

Answer 22

Electronegativity is the ability of an atom to attract the shared pair(s) of electrons in a covalent bond

Question 23

[Definition] Disproportionation

Answer 23

Disproportionation is a redox reaction in which one species is simultaneously oxidised and reduced.

Question 24

[SOP] Balancing half-equations

Answer 24

1. Balance all atoms other than H and O
2. Add H2O to balance O
3. Add H+ to balance H
4. Add electrons to balance total charge
5. Multiply each equation to equate the charges
6. Add the 2 half-equations to form the ionic equation

*** IF IT IS ALKALINE MEDIUM, CANNOT HAVE H+
SAME FOR ACIDIC MEDIUM, CANNOT HAVE OH-

Question 25

[SOP] Determining oxidation number using half-equation

Answer 25

Concept: total e- released = total e- gained
1. Mole ratio of reagents
2. Mole ratio between e- & reagent of known half eqn
3. Mole ratio between e- & reagent of unknown half eqn

Question 26

3 Common Redox Titrations

Answer 26

Manganate (VII) titration
Dichromate (VI) titration
Iodine-thiosulfate titration

Question 27

For manganate (VII) titrations, Why is H2SO4(aq) used to provide the acidic medium, not HNO3(aq) or HCl(aq)?

Answer 27

HNO3 is an oxidising agent, MnO4- also oxidising agent, so cannot hmmmm?

Cl- will be oxidised by MnO4- to form Cl2, so cannot also

Question 28

For dichromate (VI) titrations, what can be used to provide the acidic medium?

Answer 28

Both H2SO4 and HCl (dichromate is a weaker oxidising agent, so won’t be able to oxidise Cl- to Cl2)

HNO3??