Definition of Molecular Mass
Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quantity relative molecular mass, as defined by IUPAC, is the ratio of the mass of a molecule to the unified atomic mass unit (also known as the Dalton) and is unitless.
The molecular mass and relative moleculars mass are distinct from but related to the molar mass. The molar mass is defined as the mass of a given substance divided by the amount of a substance and is expressed in g/mol. The molar mass is usually the more appropriate figure when dealing with macroscopic (weighable) quantities of a substance.
More Definitions of Molecular Mass
Here are some more definitions to understand clearly:
The definition of molecular weight is most authoritatively synonymous with molecular’s mass. However, in common practice, it is also highly variable, as are the units used in conjunction with it.
Many common preparatory sources use g/mol and effectively define it as a synonym of molar mass, while more authoritative sources use Da or u and align its definition more closely with the molecular mass.
Even when the molecular weight is used with the units Da or u, it is frequently as a weighted average similar to the molar mass but with different units.
In molecular biology, the weight of macromolecules is referred to as their molecular weight and is expressed in kDa, although the numerical value is often approximate and representative of an average.
The terms molecular mass, molecular weight, and molar mass are often used interchangeably in areas of science where distinguishing between them is unhelpful.
In other areas of science, the distinction is crucial. The molecular’s mass is more commonly used when referring to the mass of a single or specific well-defined molecule and less commonly than molecular’s weight when referring to a weighted average of a sample.
Prior to the 2019 redefinition of SI base units, quantities expressed in Dalton’s (Da or u) were numerically equivalent to otherwise identical quantities expressed in the units g/mol and were thus strictly numerically interchangeable. After the 20 May 2019 redefinition of units, this relationship is only nearly equivalent.
The molecular mass of small to medium size molecules, measured by mass spectrometry, can be used to determine the composition of elements in the molecule.
The molecular masses of macromolecules, such as proteins, can also be determined by mass spectrometry; however, methods based on viscosity and light-scattering are also used to determine molecule mass when crystallographic or mass spectrometric data are not available.
The sum of the atomic masses of all atoms in a molecule, based on a scale in which the atomic masses of hydrogen, carbon, nitrogen and oxygen are 1, 12, 14, and 16, respectively.
For example, the molecular mass of water, which has two hydrogen atoms and one oxygen atom, is 18 (i.e., 2 + 16)—also called molecular weight.
Because the mass is relative to carbon-12, calling the value “relative molecular mass” is more correct. A related term is a molar mass, which is the mass of 1 mol of a sample. Molar mass is given in units of grams.
Molecular weight, also called molecular mass, is the mass of a molecule of a substance, based on 12 as the atomic weight of carbon -12.
It is calculated in practice by summing the atomic weights of the atoms making up the substance’s molecular formula. The molecular weight of a hydrogen molecule (chemical formula H2) is 2 (after rounding off); for many complex organic molecules (e.g., proteins, polymers), it may be in the millions.
The molar mass of a compound is simply the mass of the number of molecules of the compound. Between 1971 and 2019, SI defined the “amount of substance” as a separate dimension of measurement.
Therefore the mole was defined because of the amount of substance that has as many constituent particles as the number of atoms.
Since 2019, a mole of any substance contains a precisely defined number of particles, N = 6.02214076×1023. In this chapter, we will learn more about the molar mass formula, the calculation of molar mass, and various examples.
Formula of Molecular Mass
M = m / n
M = molar mass
m = mass of a substance (in grams)
n = number of moles of a substance
Molar mass = mass/mole = g/mol
This leads to two important facts. The mass of one atom of carbon-12 the atomic mass of carbon-12 is exactly 12 atomic mass units.
The mass of one mole of carbon-12 atoms is exactly 12 grams; its molar mass is exactly 12 grams per mole.
Calculation of Molecular Mass
Molecular mass may be calculated by taking the atomic mass of each element present and multiplying it by the number of atoms of that element in the molecular formula. Then, the number of atoms of each element is added together.
For example. to find the molecular mass of methane, CH4, the first step is to look up the atomic masses of carbon C and hydrogen H using a periodic table:
carbon atomic mass = 12.011
hydrogen atomic mass = 1.00794
Because there is no subscript following the C, you know there is only one carbon atom present in methane. The subscript 4 following the H means there are four atoms of hydrogen in the compound. So, adding up the atomic masses, you get:
the methane molecular mass = sum of carbon atomic masses + sum of hydrogen atomic masses
= 12.011 + (1.00794)(4)
methane atomic mass = 16.043
This value may be reported as a decimal number or as 16.043 Da or 16.043 amu.
Note the number of significant digits in the final value. The correct answer uses the smallest number of significant digits in the atomic masses, which in this case is the number in the atomic mass of carbon.
The molecular mass of C2H6 is approximately 30 or [(2 x 12) + (6 x 1)]. Therefore the molecule is about 2.5 times as heavy as the 12C atom or about the same masses as the NO atom with a molecular mass of 30 or (14+16).
Calculation of Molar Mass
Molar mass is the mass of a given substance divided by that amount, measured in g/mol.
For example, the atomic mass of titanium is 47.88 amu or 47.88 g/mol. In 47.88 grams of titanium, there is one mole or 6.022 x 1023 titanium atoms.
The characteristic molar mass of an element is simply the atomic mass in g/mol. However, molar mass can also be calculated by multiplying the atomic mass in amu by the molar mass constant (1 g/mol).
To calculate the molar mass of a compound with multiple atoms, sum all the atomic mass of the constituent atoms.
For example, the molar mass of NaCl can be calculated for finding the atomic mass of sodium (22.99 g/mol) and the atomic mass of chlorine (35.45 g/mol) and combining them. The molar mass of NaCl is 58.44 g/mol.
It’s easy to find the molecular mass of a compound with these steps:
- Determine the molecular formula of the molecule.
- Use the periodic table to determine the atomic mass of each element in the molecule.
- Multiply each element’s atomic mass by the number of atoms of that element in the molecule. This number is represented by the subscript next to the element symbol in the molecular formula.
- Add these values together for each different atom in the molecule.
The total will be the molecular mass of the compound.
Example of Simple Molecular Mass Calculation
For example, to find the molecular mass of NH3, the first step is to look up the atomic masses of nitrogen (N) and hydrogen (H).
H = 1.00794
N = 14.0067
Next, multiply the atomic mass of each atom by the number of atoms in the compound. There is one nitrogen atom (no subscript is given for one atom). There are three hydrogen atoms, as indicated by the subscript.
molecular mass = (1 x 14.0067) + (3 x 1.00794)
= 14.0067 + 3.02382 = 17.0305
Note the calculator will give an answer of 17.03052, but the reported answer contains fewer significant figures because there are six significant digits in the atomic mass values used in the calculation.
Example of Complex Molecular Mass Calculation
Here’s a more complicated example: Find the molecular mass (molecular weight) of Ca3(PO4)2.
From the periodic table, the atomic masses of each element are:
Ca = 40.078
P = 30.973761
O = 15.9994
The tricky part is figuring out how many of each atom are present in the compound. There are three calcium atoms, two phosphorus atoms, and eight oxygen atoms. How did you get that? If part of the compound is in parentheses, multiply the subscript immediately following the element symbol by the subscript that closes the parentheses.
molecular mass = (40.078 x 3) + (30.97361 x 2) + (15.9994 x 8)
= 120.234 + 61.94722 + 127.9952
= 310.17642 (from the calculator)
total molecular mass = 310.18
The final answer uses the correct number of significant figures. In this case, it’s five digits (from the atomic mass for calcium).