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Calculations
~ Unit 1 and Unit 2 ~
Mass Spectrometry Calculation
(Abundance1x m/z + (Abundance
1) 2x m/z + (Abundance
2) 3x m/z3)



Total Abundance

Example 1
Find the relative atomic mass of iron using the following data:
54 56 57 58
Fe (5.8%) Fe (91.6%) Fe (2.2%) Fe (0.3%)…

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Protons =6 Neutrons =9 Electrons =6


Moles and Avogadro's Constant
A mole is the number of particles, the number of moles can be calculated by using the following
equation/triangle. This can also be used to calculate the mass (in grams) and the Mr of a compound.

Avogadro's Constant
The definition…

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0.500 / 0.500 =
1.00 mol dm-3

Example 2


25cm3 of
0.1 mol dm-3 NaOH neutralised
18.5cm3 of HCl. Calculate the concentration of HCl
NaOH + HCl NaCl +H 2O



Moles NaOH: (25/1000) x 0.1
= 0.0025

Moles HCl: 0.0025

Concentration HCl: 0.0025 / (18.5/1000)
= 0.135 mol dm-3

Example…

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Mg
3N 2




Molecular Formula
The molecular formula is the total number of atoms of each element in each molecule of a substance

Example
Calculate the molecular formula of CHwhen the relative molecular mass is 78

1. Take the empirical formula and Mr that is given
EF: CH
Mr: 78…

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Amount of product (actual yield)

Theoretical Yield

Reasons for the Yield being under 100%
Reactant could be left on equipment
Unexpected reactions
Contaminated reactants or impurities in the reactants
The reaction is reversible


Atom Economy
This is the proportion of the products of a reaction compared to the desired products.…

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Q is the enthalpy change- KJ
m is the mass of water/solution

c is the specific heat capacity of water
T is the change in temperature


Example

Calculate the neutralisation enthalpy of 50cm3 of 1 mol dm-3
NaOH with 50cm3 of 1 mol dm-3 HCl and the temperature
change of…

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H=
-75


Enthalpy of Combustion and Formation
This calculation uses both enthalpies of formation and combustion to find the H, the diagram below is used to break
the calculation down:




Example
Calculate the enthalpy of combustion of propane (C3H8 (g)) given the following enthalpy changes
Hc

C (s)= -393 H…

Page 8

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H = 76


Oxidation States
To calculate oxidation states you have to take into account the following `rules'

1. On simple ions the oxidation state is the charge on the ion
2. In elements the oxidation state is always 0
3. The total of all the oxidation states needs to…

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