Question

Short note on molecular mass

Answer 1

Molecular mass is the sum of the atomic masses in a molecule. Lawrence Lawry, Getty Images
by
Anne Marie Helmenstine, Ph.D.
Updated April 05, 2018
In chemistry, there are different types of mass. Often, the terms are called weight rather than mass and used interchangeably. A good example is molecular mass or molecular weight.

Molecular Mass Definition
Molecular mass is a number equal to the sum of the atomic masses of the atoms in a molecule. The molecular mass gives the mass of a molecule relative to that of the 12C atom, which is taken to have a mass of 12.


Molecular mass is a dimensionless quantity, but it is given the unit Dalton or atomic mass unit as a means of indicating the mass is relative to 1/12th the mass of a single atom of carbon-12.

Also Known As
Molecular mass is also called molecular weight. Because the mass is relative to carbon-12, it's more correct to call the value "relative molecular mass".

A related term is molar mass, which is the mass of 1 mol of a sample. Molar mass is given in units of grams.

Sample Molecular Mass Calculation
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:

methane molecular mass = sum of carbon atomic masses + sum of hydrogen atomic masses

methane molecular mass = 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 mass as the NO atom with a molecular mass of 30 or (14+16).

Problems Calculating Molecular Mass
While it's possible to calculate molecular mass for small molecules, it's problematic for polymers and macromolecules because they are so large and may not have a uniform formula throughout their volume. For proteins and polymers, experimental methods may be used to obtain an average molecular mass. Techniques used for this purpose include crystallography, static light scattering, and viscosity measurements.


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Molecular weight is the total mass of a single molecule.

Answer 2

......... Now get your answer......
Molecular mass is also

known as molecular

weight. It is the quantity

of matter contained in

substance that is called

molecular mass or weight.

This lesson introduces the

definitions of molecular

mass and molar mass.

Using information from

the periodic table, you'll

learn how to use the definitions to calculate either quantity.

Molecules, Moles, Mass & Weight

Before we try calculations involving atoms and molecules, let's review some terms and units. Some sources make a distinction between molecular mass and molecular weight, but the difference is pretty subtle and more relevant when you're studying atomic structure or nuclear chemistry. For now, we'll treat them as interchangeable.

There is one set of terms that you need to treat as distinct: molecular mass and molar mass. We need to focus on the difference because they have different units. Molecular mass is the mass of a single molecule, given in atomic mass units (amu). Molar mass is the mass of one mole of a substance, given in units of grams per mole (g/mole).

There's one more term we need to consider, for the sake of scientific accuracy: not all compounds are molecules. An ionic compound like sodium chloride is not a molecule, because it's not held together by covalent bonds. So, can we talk about the molecular mass or molecular weight of sodium chloride? Not really, but lots of people do. When discussing ionic compounds, a better term to use is formula weight or formula mass.

Add Them Up

To understand the definition and calculation of molecular mass, let's first look at something we're a little more familiar with: people. Suppose you wanted to know the weight of a whole family. It's pretty easy to figure out how much each person weighs. But it's safe to say that it would be hard to get everyone on a scale all at once, even if it went that high. We can calculate the total weight if we know the weight of each individual person. Let's suppose that the two babies each weigh 8 pounds, the girls each weigh 50 pounds, mom weighs 130 pounds, and dad weighs 160 pounds. We can calculate the total like this:

2 babies (8 lbs/baby) + 2 girls (50 lbs/girl) + 130 lbs + 160 lbs = 406 lbs

Even though atoms weigh a whole lot less than people, we can use the same approach to calculate the weight of a molecule.

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