Molar Mass

The molar mass of a substance is the sum of the mass of all the atoms that comprise a mole of the given substance.

The unit of molar mass is grams per mole.

The Concept of Molar Mass

Chemistry is intriguing. There exist many terms and definitions for studying a substance and its chemical behavior. Atoms, electrons, protons, moles, molecules, compounds these define the property and behavior of every substance in nature.

As we all know, an atom is the basic component of any substance. Atoms are made of protons, neutrons, and electrons. While the first two comprise the nucleus of the atom, electrons keep revolving around the nucleus in a cloudy mass like formation.

“Protons give an atom its identity, electrons its personality.”
― Bill Bryson

Though atom is the fundamental unit of a substance, in chemistry, it is the term ‘mole’ that is used for all calculations. Why? A mole is a standard unit which is used to compare any two compounds or substances and helps in performing calculations easily. This concept of the mole is like a bridge between atomic chemistry and the physical chemistry that chemists deal in laboratories.

A mole is the amount of a given substance that contains 6.023 x 1023 of its particles. The concept of molar mass eases comparison of two chemical substances. We cannot say heavier item or denser item when it comes to chemistry and reactions. This sounds little out of concept while dealing with intricate reactions. This is where a uniform terminology surrounded by moles comes into great help.

Hence, the reason we talk about ‘molar’ mass of a given chemical substance. Though the measurement of chemical substance involves grams, the final unit of molar mass is grams per mole.

The knowledge of molar mass is quintessential in studying the properties and reactions involved with various chemical compounds.

Calculation of Molar Mass

Simply stating, molar mass can be calculated by adding the atomic mass of the total number of individual elements present in the given compound.

The steps involved in the calculation of molar mass are given below:

• Step 1: Break down the formula of the compound into individual elements.
• Step 2: Look up into periodic table and get the atomic masses of individual elements.
• Step 3: In case, any element is present more than once in the compound, multiply its atomic mass by that number.
• Step 4: Add all the individual masses, thus obtained to arrive at the final molar mass of the substance.

Let us understand the calculation of molar mass through some examples:

Example 1: Calculate the molar mass of NaCl.

• Step 1: NaCl has one atom of sodium and one atom of Chlorine.
• Step 2: From the periodic table, the atomic mass of Na is 22.98976 grams per mole and the atomic mass of Cl is 35.453 grams per mole.
• Step 3: There are no multiples of either Na or Cl in the formula of NaCl. So, move on to the next step.
• Step 4: Add the individual atomic masses of Na and Cl, which gives 58.44276 grams per mole, which is the molar mass of NaCl.

Example 2: Calculate the molar mass of sucrose (C₁₂H₂₂O₁₁).

• Step 1: Sucrose has 12 atoms of Carbon, 22 atoms of Hydrogen and 11 atoms of Oxygen.
• Step 2: From the periodic table, the atomic masses of C is 12.0107 grams per mole, H is 1.00794 grams per mole and O is 15.9994 grams per mole.
• Step 3: Since there are multiple atoms of each element, we need to multiply them accordingly:
  i.e., 12 × 12.0107 + 22 × 1.00794 + 11 × 15.9994
• Step 4: Add the individual atomic masses of C, H, and O multiplied with their number of atoms accordingly, then you get 342.297 as the molar mass of Sucrose.

The beauty of a living thing is not the atoms that go into it, but the way those atoms are put together.
― Carl Sagan

Applications of Molar Mass

Molar mass meets the scientific needs of calculations of chemists and physicists with ease.

To Compare Experimental Results

For example, we have two different substances. Their number of moles are the same, but the volume they occupy is different. Then, we can easily infer that the moles of the substance that occupies higher volume is bigger in size than the other substance.

To determine the contribution of individual elements in a compound

Using molar mass, you can understand the contribution of a given element in the overall compound. For example, consider 28grams of carbon monoxide. If you want to check the contribution of carbon to the total molar mass, you can do it as: molar mass of carbon divided by the total weight of the sample of carbon monoxide multiplied by 100. i.e., 12/28 *100 = 42.89 %.

Finding the theoretical molar mass of complex compounds

In case, you are given a complex compound, you can easily determine its molar mass without actually experimenting on it or making heavy calculations. By directly adding up the molar masses of each element present in the formula, you arrive at the total molar mass of the substance.

The formula for calculation of molar mass for liquids and solutions:

For liquids, the concentration C of the solution is given by the ratio of the number of grams ‘n’ of solute in the total volume ‘V’ of the solution.

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