Molar Mass – concepts and formula

Last updated on October 30th, 2021 at 05:05 am

The mass of one mole of a substance in grams is called its molar mass. The molar mass in grams is numerically equal to atomic/molecular/formula mass in u.

The molar mass of water = 18.02 g mol^-1
The molar mass of sodium chloride = 58.5 g mol^-1

Molar mass is the mass of a single mole. In other words, Molar mass is the mass of a given substance divided by the number of moles.

Hence we can write that, Molar mass (M) = Mass of substances (m ) / number of moles (n)
=> M= m/n

Calculating molar mass from atomic mass

Here we will see how to calculate the molar mass of an element and a compound with some examples.

how to calculate the molar mass of an element? | molar mass of titanium

The characteristic molar mass of an element is numerically equal to the atomic mass with a unit g/mol.

For example, the atomic mass of titanium is 47.88 u. So, the molar mass of the titanium element is 47.88 g/mol.
This means, in 47.88 grams of titanium, there is one mole or 6.022 x 10²³ titanium atoms.
In other words, the mass of one mole or 6.022 x 10²³ titanium atoms is 47.88 grams.

how to calculate the molar mass of a compound? | molar mass of NaCl

To calculate the molar mass of a compound with multiple atoms, the molecular mass of the compound (i.e., the sum of all the atomic mass of the constituent element atoms) is to be considered.

For example, let’s see how to find out the molar mass of NaCl.

The atomic mass of sodium(Na) is 22.99 u and that of chlorine(Cl) is 35.45 u. The sum of these two atomic masses is 58.44 u.
In other words, the molecular mass of NaCl is 58.44 u.
Hence, the molar mass of NaCl is 58.44 g/mol.

Formula of Molar mass of a gaseous substance in terms of its density

Let’s see how to express the Formula of Molar mass of a gaseous substance in terms of its density.

The molar mass of an ideal gas can be calculated from the ideal gas equation as :

pV = nRT …………….. (1)

Now, number of moles (n) = Mass of substances (m ) / Molar mass (M)
n = m/M ………………….(2)

Now, from equations (1) and (2) we get:

pV = nRT

or pV=(m/M) RT

or pV = (mRT)/M

or M = (mRT) / (pV) = dRT/p,

M = molar mass = dRT/p

[ where, m/V = mass/volume = density = d ]