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Definitions of mass in Chemistry

The following are common mass-related terminologies that students encounter when they study Chemistry:-

Relative isotopic mass

The relative isotopic mass is the ratio of the mass of an isotope in unified atomic mass unit to one unified atomic mass unit. It is a dimensionless quantity, for example, the relative isotopic mass of 1H is 1.007825.

Relative atomic mass

The relative atomic mass of an element is the weighted average of the relative isotopic masses of all its isotopes. It is represented by the symbol Ar and is the value usually found next to the symbol of an element in a periodic table. For example, the relative atomic mass of copper, Ar(Cu), is (62.929601 x 0.6917) + (64.927794 x 0.3083) ≈ 63.546.

Relative isotopic mass % Abundance
63Cu 62.929601 69.17
65Cu 64.927794 30.83

The terms ‘atomic weight’ and ‘standard atomic weight’ are sometimes used in place of relative atomic mass. Strictly speaking, standard atomic weight is only equivalent to relative atomic mass when studying the masses of elements on earth.

 

Relative molecular mass

The relative molecular mass of a covalent bonded molecule is the sum of the relative atomic masses of all atoms making up the molecule. It is represented by the symbol Mr . For example, the relative molecular mass of carbon dioxide, Mr(CO2), is 12.011 + (15.999 x 2) = 44.009.

Relative atomic mass
C 12.011
O 15.999

For an ionic compound, the term relative formula mass is used instead of relative molecular mass.

 

Atomic mass

The mass of an atom (i.e. the mass of an isotope and not an average mass of all isotopes of an element). It is defined in unified atomic mass unit, u, and can be converted to the basic SI mass unit, kg. When the atomic mass of an isotope is stated in u, it has exactly the same numerical value as the isotope’s relative isotopic mass. For example,

Relative isotopic mass Atomic mass, u Atomic mass, kg
2H 2.014104 2.014104 3.34450 x 10-27

The relationship between the unified atomic mass unit, u, and the basic SI mass unit, kg, is:

1u=\frac{0.001}{N_{A}}kg

where is NA is the Avogadro constant. The value 0.001 kgmol-1 is called the molar mass constant and is usually written as 1 gmol-1. With the new definition of the Avogadro constant as exactly 6.02214076 x 1023 mol-1, the value of the molar mass constant deviates very slightly from the exact value of 1 gmol-1 and has to be determined through future experiments.

 

Molecular mass

The molecular mass is the mass of a molecule. It is defined in unified atomic mass unit, u, and can be converted to the basic SI mass unit, kg. Since an atom in a molecule is a specific isotope of an element, different molecules of the same compound may have different molecular mass. For example, the molecular mass of nitrogen gas can be

Molecular mass, u
14N14N 14.003074 + 14.003074 = 28.006148
14N15N 14.003074 + 15.000109 = 29.003183
15N15N 15.000109 + 15.000109 = 30.000218

For ionic compounds, the term formula mass is used instead of molecular mass.

 

Molar mass

The molar mass of a substance is the mass of the substance per mole. It is represented by the symbol M and has the SI unit of kgmol-1. However, most molar masses are expressed in gmol-1 for practical purpose. For an isotope, it is its atomic mass in gmol-1. For an element, it is its average atomic mass in gmol-1. For a covalent and an ionic compound, it is the average molecular mass in gmol-1 and average formula mass in gmol-1 respectively. For example,

M, gmol-1
35Cl 34.97
Cl 35.45
N2 28.014
CuSO4 159.602
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How to ‘weigh’ an atom?

How do you weigh an atom?

As mentioned in the earlier sections, the mass of an atom is measured on the unified atomic mass unit scale. This is carried out in a mass spectrometer with carbon-12 as a standard reference. For example, mass spectrometric data for the ratio of the mass-to-charge ratio (u/z) of 2H to that of 12C is 0.167842. Thus, the mass of 2H on the carbon-12 unified atomic mass unit scale is:

one\; ^{2}H=\frac{\frac{u}{z}\; of\; one\; ^{2}H}{\frac{u}{z}\; of\; one\; ^{12}C}\times one\; ^{12}C\; \; \; \; \; \; \; \; (6)

one\; ^{2}H=0.167842\times 12\: u=2.014104\: u

Using eq2, the mass of one 2H is 2.014104 x 1.660539 x 10-27 = 3.3450 x 10-27 kg.

The conversion from u to kg is based on eq2, which is dependent on the uncertainty in the Avogadro constant prior to Nov 2018. However, the molar mass of 2H, which is equal to 2.0141 g, is not. This is shown by substituting eq1 in eq6,

one\; ^{2}H=\frac{\frac{u}{z}\; of\; one\; ^{2}H}{\frac{u}{z}\; of\; one\; ^{12}C}\times \frac{0.012\: kg}{N_{A}}

Multiplying both sides by NA,

N_{A}\times one\; of\; ^{2}H=\frac{\frac{u}{z}\; of\; one\; ^{2}H}{\frac{u}{z}\; of\; one\; ^{12}C}\times 0.012\: kg

one\;mole\; of\; ^{2}H\left ( molar\; mass\; of \; ^{2}H \right )=\frac{\frac{u}{z}\; of\; one\; ^{2}H}{\frac{u}{z}\; of\; one\; ^{12}C}\times 0.012\: kg\; \; \; \; \; \; \; \; (7)

where the RHS of eq7 is independent of NA.

This is why the molar mass of another isotope, silicon-28, is used in determining the Avogadro constant in X-ray diffraction experiments. Furthermore, it is simpler to present the mass of an atom or isotope in the form of relative isotopic mass, which is a dimensionless quantity defined as the ratio of the mass of an isotope in unified atomic mass unit to one unified atomic mass unit. The table below lists the relative masses of isotopes of the first few elements in the periodic table:

Atomic no. (Z) Mass no. (A) Symbol Relative isotopic mass*
1 1 1H 1.007825
  2 2H 2.014104
2 3 3He 3.016029
  4 4He 4.002603
3 6 6Li 6.015122
4 9 9Be 9.012182
5 10 10B 10.012937
  11 11B 11.009305
6 12 12C 12.000000
8 16 16O 15.994915
  17 17O 16.999132
  18 18O 17.999160
9 19 19F 18.998403
10 20 20Ne 19.992440
  21 21Ne 20.993847
  22 22Ne 21.991386

*With the new definition of the Avogadro constant, the mass of an atom in kg is no longer subject to the uncertainty of the Avogadro constant, but is contingent on the uncertainty in the value of the molar mass constant, since 1u=\frac{M_u}{N_A}g. However, the relative masses of isotopes remain unchanged. 

 

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Mole concept – What are its applications?

Having understood the mole concept, let’s see how it can be applied to solve problems.

Consider the following chemical equation, which is the combustion of methane to give carbon dioxide and water:

CH_{4}\left ( g \right )+2O_{2}\left ( g \right )\rightarrow CO_{2}\left ( g \right )+2H_{2}O\left ( l \right )\; \; \; \; \;\; \; \; (3)

It clearly states that 1 molecule of methane reacts with 2 molecules of oxygen to give 1 molecule of carbon dioxide and 2 molecules of water. Since the reactant ratio of 1:2 gives the product ratio of 1:2, we can say that 1 mole of methane reacts with 2 moles of oxygen to give 1 mole of carbon dioxide and 2 moles of water if we want the equation to depict the reaction from a macroscopic point of view. For convenience, you may regard all chemical equations as if they are written from a macroscopic perspective, i.e. the numbers in front of chemical formulas in all chemical equations are in moles (molar format). Note that the number ‘1’ is not added in front of CH4 and CO2 as it is redundant. The letters in brackets signify the physical states of the molecules: (g) for gaseous, (l) for liquid, (s) for solid and (aq) for aqueous.

 

Question 1

With reference to eq3, how many moles of CO2 are produced from 1 mole of CH4 and 1.5 moles of O2?

Answer 1

From eq3, 2 moles of O2 react with 1 mole of CH4 to give 1 mole of CO2 and 2 moles of water. So, 1.5 moles of O2 must react with only 1.5/2 moles of CH4 to give 1.5/2 moles of CO2 and 1.5 moles of water. There will be an excess of 1 – (1.5/2) = 0.5/2  moles of CH4 that is unreacted. Note that O2 is the limiting reactant in this example, i.e. the chemical that is totally consumed when the reaction is complete.

 

Let’s try another one.

Question 2

How many oxygen atoms are there in two moles of sulphate ions, SO42-, and how many moles of SO42- are there in 5g of CaSO4?

Answer 2

In one ion of SO42-, there are 1 atom of sulphur and 4 atoms of oxygen (note that the superscript “2-” refers to the charge of the ion). In one mole of SO42-, there are 1 mole of sulphur and 4 moles of oxygen. Using eq1,

number (of atoms) = number of moles x 6.02 x 1023

number (of oxygen atoms) = 2 x 4 x 6.02 x 1023

Therefore, there are 4.82 x 1024 atoms of oxygen in two moles of SO42-. 1 mole of CaSO4 contains 1 mole of SO42-. Using eq2 where n=\frac{m}{M}, the number of moles of SO42- in 5g of CaSO4 is:

n=\frac{5}{40.1+32.1+\left ( 16.0\times 4 \right)}=3.67\times 10^{-2}\: moles

 

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