TOPIC 1: MOLE CONCEPT & STOICHIOMETRY Flashcards
[Definition] Isotopes
Isotopes are atoms of the same element whose nuclei have the same number of protons but different number of neutrons.
[Definition] Relative Isotopic Mass
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
[Definition] Relative Atomic Mass
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
[Definition] Relative Molecular Mass
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
[Definition] Relative Formula Mass
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
[Definition] Mole
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)
[Formula] 4 Main Formula
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)
[Conversion] Volumes
1 dm3 = 1000 cm3 = 0.001m3 = 1000ml = 1litre
[Definition] Empirical Formula
Empirical formula is the formula that shows the simplest whole number ratio of the number of atoms of each element present in the substance.
[Definition] Molecular Formula
Molecular formula is the formula that shows the actual number of atoms of each element in one molecule of the compound.
[SOP] Finding empirical / molecular formula
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 …
[Formula] Percentage Yield
% Yield = actual mass / theoretical mass x 100%
% Yield = actual amt (in mol) / theoretical amt (in mol) x 100%
[SOP] Solving combustion reaction questions
- Write out formula - CxHy + (x+y/4)O2 –> xCO2 + (y/2)H2O
- Based on volume of CxHy, find stoichiometric ratios of the rest
- Solve based on volumes provided by question
Difference between equivalence point and end point
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
What’s the difference between a strong and weak acid/base reaction in titration?
The ending pH of the solution
BOTH strong and weak acid/base WILL REACT COMPLETELY in titration reactions !!!
[Formula] Dilution of solutions
c1V1 = c2V2
*Concept: amt of substance (mol) is the same before and after dilution
[Definition] Redox Reaction
A reaction in which oxidation and reduction take place at the same time
Oxidation VS Reduction
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
MnO4- in acidic and neutral/alkaline medium
[Acidic] MnO4- to Mn2+ (pale pink / colourless if in dilute solution)
[Neutral/alkaline] MnO4- to MnO2 (brown solid)
[SOP] Order of assigning oxidation numbers (standard)
Group 1 (always +1)
Group 2 (always +2)
Aluminium (+3)
F (-1)
O (-2)
H (+1)
[SOP] Order of assigning oxidation numbers (complicated molecules)
- More electronegative atom gets the negative sign, the other gets the positive sign
- Single bond, +/-1 ; Double bond, +/-2 ‘ Trible bond, +/-3
- Average oxidation number = sum of oxidation numbers / number of atoms
[Definition] Electronegativity
Electronegativity is the ability of an atom to attract the shared pair(s) of electrons in a covalent bond
[Definition] Disproportionation
Disproportionation is a redox reaction in which one species is simultaneously oxidised and reduced.
[SOP] Balancing half-equations
- Balance all atoms other than H and O
- Add H2O to balance O
- Add H+ to balance H
- Add electrons to balance total charge
- Multiply each equation to equate the charges
- 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-
[SOP] Determining oxidation number using half-equation
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
3 Common Redox Titrations
Manganate (VII) titration
Dichromate (VI) titration
Iodine-thiosulfate titration
For manganate (VII) titrations, Why is H2SO4(aq) used to provide the acidic medium, not HNO3(aq) or HCl(aq)?
HNO3 is an oxidising agent, MnO4- also oxidising agent, so cannot hmmmm?
Cl- will be oxidised by MnO4- to form Cl2, so cannot also
For dichromate (VI) titrations, what can be used to provide the acidic medium?
Both H2SO4 and HCl (dichromate is a weaker oxidising agent, so won’t be able to oxidise Cl- to Cl2)
HNO3??
