Calculations

All the calculations with examples

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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%)
Ar =((54 x 5.8) + (56 x 91.6) + (57 x 2.2) + (58 x 0.3))
(5.8 + 91.6 + 2.2 + 0.3)
= 55.91 56
Example 2
A sample of sulphur consisting of three isotopes has a relative atomic mass of32.16 . The following data gives
information about the relative abundance of two of the isotopes:
32 33
S (91.0%) S (1.8%)
Use this information to find the relative abundance and hence the mass of the third isotope
32.16 = ((32 * 91) + (33 * 1.8) + (x * (100-91-1.8)))
100
3216 = 2971.4 + 7.2x
7.2x= 244.6
x= 244.6
7.2
= 33.97 34
Mass Number and Relative Atomic Mass
The mass number is the top number of an element in the periodic table. This can be used to calculate the number of
neutrons. The bottom number situated on an element is the relative atomic mass and is also known as the proton
number.
Example
Calculate the number of protons, electrons and neutrons in 15 6C
Relative atomic mass = No. of protons = No. of electrons = 6
Neutrons = mass ­ atomic mass = 15 ­ 6 = 9

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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 of the Avogadro's Constant/Avogadro's Number is the number of atoms in 12g of Carbom-12.
Avogadro's number = 6.…read more

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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 3
Calculate the concentration of H
2SO 4if
20.0cm3 were neutralized by
19cm3 of
0.5 mol dm-3 Ca(OH)2
Ca(OH)2 + 2HCl CaCl 2+ 2H 2O
Moles Ca(OH)2: (19/1000) x 0.5
= 0.0095
Moles HCl: 1:2 ratio
= 0.0095 x 2
= 0.…read more

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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
2. Calculate the sum of the Mr
(12 + 1) n = 78
3.…read more

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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. It calculated the waste and the
higher the atom economy, the less waste is produced.…read more

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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 6.6K. (The specific heat capacity of water is 4.2)
Q = 100 x 4.2 x 6.6
= 2772
No. of moles: 1 x (50 / 1000)
= 0.05
1 mole gives: 2772 / 0.…read more

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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 2 (g)= -286 Hf
C
3H8= -103
H = [ of Hf reactants ] - [ of Hf products ]
C
3H8+ 5O2 3CO2+ 4H2O
3C(s)+ 4H2(g)
H = 103 + (-393 x 3) +…read more

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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 equal the overall charge
4. In molecules and more complex ions the more electronegative element is assumed to be the negative ion
5.…read more

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