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Brown's Chemistry  GCSE/IGCSE/GCE (basic A level)
O Level
Online Chemical Calculations
2.
Calculating relative
formula mass or relative molecular mass RFM or M_{r}
This page describes, and
explains, with worked out examples, the method of how to calculate the relative
formula mass of a compound (ionic or covalent) or the relative molecular mass of an element
or a covalent compound.
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2.
How to calculate relative
formula mass or relative molecular mass RFM/RMM or M_{r}
How do I calculate relative molecular mass? RMM
How to calculate relative formula mass? RFM
Is there any difference between RMM and RFM?
Does it matter whether the compound is ionic or
covalent?
If all the individual atomic masses of all the atoms in a formula are added together you have calculated the
relative formula mass (for ionic compounds e.g. NaCl = 58.5) or molecular
mass (for covalent elements e.g. N_{2} = 28 or compounds e.g. C_{6}H_{12}O_{6}
= 180).
To be honest, the term relative formula mass can be used with any
compound whether it be ionic or covalent  it just seems not quite correct to
talk about the molecular mass of an ionic compound when it doesn't consist of
molecules!
The shorthand M_{r} can be used for
the formula of any element or compound and to repeat, 'it doesn't matter
whether a compound is ionic or covalent'.
M_{r} = Relative formula mass =
relative molecular mass = the sum of all the atomic masses for all the atoms
in a given formula
Whereas relative atomic
mass (section 1. Relative Atomic Mass) only applies to a single atom but anything with at least two
atoms requires the term relative formula mass or relative molecular mass.
The
most common error is to use atomic/proton numbers instead of atomic masses,
unfortunately, except for hydrogen, they are different!
Examples
of relative formula/molecular mass calculations:
How to calculate relative molecular mass = How
to calculate relative formula mass
Recap: Molecular/formula mass = total of
all the atomic masses of all the atoms in the molecule/compound.
 Molecular/formula mass calculation Example 2.1
 The diatomic molecules of the elements hydrogen H_{2} and
chlorine Cl_{2}
 relative atomic masses, Ar:
H = 1, Cl = 35.5
 Formula masses, RMM or M_{r},
are H_{2} = 2 x 1 = 2, Cl_{2} = 2 x
35.5 = 71 respectively.
 Molecular/formula mass calculation Example 2.2
 The element
phosphorus
consists of P_{4} molecules.
 RMM or M_{r} of phosphorus = 4 x
its atomic mass = 4 x 31 = 124
 Molecular/formula mass calculation Example 2.3: The compound water H_{2}O
 relative atomic masses are H=1 and O=16
 RMM or M_{r} = (1x2) + 16 = 18
(molecular mass of water)
 Molecular/formula mass calculation Example 2.4
 The compound sulphuric acid H_{2}SO_{4}
 relative atomic masses are H=1, S=32 and O=16
 RMM or M_{r} = (1x2) + 32 + (4x16) =
98 (molecular mass of sulphuric acid)
 Molecular/formula mass calculation Example 2.5
 The compound calcium hydroxide
Ca(OH)_{2} (ionic)
 relative atomic masses are Ca=40, H=1 and O=16
 RMM or M_{r} = 40 + 2 x (16+1) =
74
 Molecular/formula mass calculation Example 2.6
 The ionic compound
aluminium oxide (Al^{3+})_{2}(O^{2})_{3}
or just plain Al_{2}O_{3}, but it makes no difference
to the calculation of relative formula mass or relative molecular mass.
 relative atomic masses are Al = 27 and O = 16
 so the formula mass RFM or M_{r} =
(2 x 27) + (2 x 16) = 102
 Molecular/formula mass calculation Example 2.7
 Calcium phosphate is
also ionic but a more tricky formula to work out!
 (Ca^{2+})_{3}(PO_{4}^{3})_{2}
or Ca_{3}(PO_{4})_{3}, but it makes no
difference to the calculation of relative formula mass or relative molecular
mass.
 atomic masses: Ca = 40, P = 31, O =16
 RFM or M_{r} = (3 x 40) + 3 x {31 +
(4 x 16)} = (120) + (3 x 95) = 405
 Molecular/formula mass calculation Example 2.7
 Glucose C_{6}H_{12}O_{6}
 atomic masses: C = 12, O= 16, H = 1
 Molecular mass of glucose M_{r}(C_{6}H_{12}O_{6})
= (6 x 12) + (12 x 1) + (6 x 16) = 180
Selfassessment Quizzes
[rfm] type in answer
for
F and H or multiple choice
for
F and H
OTHER CALCULATION PAGES

What is relative atomic mass?,
relative isotopic mass and calculating relative atomic mass

Calculating relative
formula/molecular mass of a compound or element molecule
(this page)

Law of Conservation of Mass and simple reacting mass calculations

Composition by percentage mass of elements
in a compound

Empirical formula and formula mass of a compound from reacting masses
(easy start, not using moles)

Reacting mass ratio calculations of reactants and products
from equations
(NOT using
moles) and brief mention of actual percent % yield and theoretical yield,
atom economy
and formula mass determination

Introducing moles: The connection between moles, mass and formula mass  the basis of reacting mole ratio calculations
(relating reacting masses and formula
mass)

Using
moles to calculate empirical formula and deduce molecular formula of a compound/molecule
(starting with reacting masses or % composition)

Moles and the molar volume of a gas, Avogadro's Law

Reacting gas volume
ratios, Avogadro's Law
and GayLussac's Law (ratio of gaseous
reactantsproducts)

Molarity, volumes and solution
concentrations (and diagrams of apparatus)

How to
do volumetric titration calculations e.g. acidalkali titrations
(and diagrams of apparatus)

Electrolysis products calculations (negative cathode and positive anode products)

Other calculations
e.g. % purity, % percentage & theoretical yield, volumetric titration
apparatus, dilution of solutions
(and diagrams of apparatus), water of crystallisation, quantity of reactants
required, atom economy

Energy transfers in physical/chemical changes,
exothermic/endothermic reactions

Gas calculations involving PVT relationships,
Boyle's and Charles Laws

Radioactivity & halflife calculations including
dating materials
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