Molar Mass and Mol Number

Molar mass is the mass contained in 1 mole of substance. The mole is the International System's unit of measurement used to determine the quantity of elementary particles.

The mole number is related to Avogadro's Constant, N_THE, which corresponds to 6.02 x 10²³ atoms, ions or molecules of a substance.

Molar mass

The molar mass has the same numerical value as the molecular mass of a substance, however, its unit is g/mol (grams per mol).

The molecular mass (MM) corresponds to the sum of the atomic masses of the atoms that make up the substance, found in the Periodic Table. Both molecular mass and atomic mass are expressed in atomic mass units (u).

See the approximate molar masses of some compounds:

• Water (H₂O): 18 g/mol
• Oxygen gas (O₂): 32 g/mol
• Sodium chloride (NaCl): 58.5 g/mol

How to calculate molar mass?

Now, to explain how to perform the calculation step-by-step, we will use ethanol, CH₃CH₂OH, as an example.

Step 1: count the number of atoms of each chemical element in the substance's formula.

the CH₃CH₂OH is formed by:

• 1 oxygen atom (O)
• 2 carbon atoms (C)
• 6 hydrogen atoms (H)

Step 2: see the Periodic table to know the atomic mass of each element of the substance.
Note: Here we will use approximate values.

• Hydrogen (H): 1 u
• Oxygen (O): 16 u
• Carbon (C): C: 12 u

Step 3: multiply the masses of the elements by the respective number of atoms in the substance.

• Oxygen (O): 1 x 16 u = 1 x 16 u
• Carbon (C): C: 2 x 12 u = 24 u
• Hydrogen (H): 6 x 1 u = 6 u

Step 4: add the masses to find the molecular mass.

MM_Ethanol: 16 u + 24 u + 6 u = 46 u

Therefore, the mass of ethanol is 46 u or 46 g/mol. This means that in one mole there is 6.02 x 10²³ molecules, which corresponds to 46 grams.

know more about molecular mass and atomic mass.

What is mol?

The mole corresponds to the number of elementary species in a given mass of a substance. One mole has an absolute value of 6.02 x 10²³.

This constant is important for performing chemical calculations, as it allows us to obtain a ratio between the atomic scale and a scale that can be measured.

For example, looking at the Periodic Table we see that the atomic mass of hydrogen is 1 u and the mass of oxygen is 16 u. Therefore, the molecular mass of water (H₂O) is 18 u.

As the molecular mass of water is 18 u, it is understood that the molar mass of water is 18 g/mol, that is, 1 mol of water has 18 g of mass.

In summary, we have: 1 mole of water = 6.02 x 10²³ molecules = 18 grams.

Learn more about Avogadro's Constant.

Relationship between the number of moles and the molar mass

Mol is a term widely used to determine quantities of particles, which can be atoms, molecules, ions, among others. The molar mass corresponds to the molecular mass of a substance, being expressed in grams per mole.

The word mol derives from moles, in Latin, which means a heap, a heap or a pile.

It is a very important term in chemistry, since in industry, for example, one does not work with few molecules but with large amounts of substances.

When the term mol is used it is referring to a clump of particles corresponding to 6.02 x 10²³. So, if we talk about 1 mole of calcium atoms, we have 6.02 x 10²³ calcium atoms.

This value refers to Avogadro's Constant, principle according to which: "equal volumes of two gases any under the same pressure and temperature conditions contain the same number of moles of molecules of gas."

Therefore, 1 mole of a substance corresponds to the molar mass of a substance and contains 6.02 x 10²³ molecules of this substance.

Read too: Molarity and Molality.

Solved Exercises

question 1

Calculate the molar mass of the following substances.

a) Carbon dioxide, CO₂
b) Hydrochloric acid, HCl
c) Glucose, C₆H₁₂O₆

See Answer

Correct answer: a) 44 g/mol, b) 36.5 g/mol and c) 180 g/mol.

To facilitate the calculations, we will use approximate values ​​for atomic masses.

a) Carbon dioxide, CO₂

Element	The amount	atomic mass	Result
Ç	1	x	12 u	=	12 u
O	2	x	16 u	=	32 u
CO molecular mass2	=	32 + 12 = 44 u

Therefore, the molar mass of carbon dioxide is 44 g/mol.

b) Hydrochloric acid, HCl

Element	The amount	atomic mass	Result
H	1	x	1 u	=	1 u
Cl	1	x	35.5 u	=	35.5 u
Molecular mass of HCl	=	1 + 35.5 = 36.5 u

Therefore, the molar mass of hydrochloric acid is 36.5 u.

c) Glucose, C₆H₁₂O₆

Element	The amount	atomic mass	Result
Ç	6	x	12 u	=	72 u
O	6	x	16 u	=	96 u
H	12	x	1 u	=	12 u
Molecular mass of glucose	=	72 + 96 + 12 = 180 u

Therefore, the molar mass of glucose is 180 g/mol.

question 2

To make some jewelry for her new collection, a designer used 39.4g of gold. Knowing that the atomic mass of gold (Au) is 197 au.m.a, calculate how many atoms were used.

See Answer

Correct answer: 1,204 x 10²³ gold atoms

We know that: 1 atom of Au = 197 au.u.m.a → 1 atom-gram (atg) of Au = 197 g → 6.02 x10²³ Au atoms

From this data, we will do it in two steps:

First step:

197 g ______ 1 atg of Au

39.4 g ______ x

197.x = 39.4.1atg → x = 39.4 atg / 197 → x = 0.2 atg of Au

Second stage:

1 atg of Au ______ 6.02 x 10²³ gold atoms

0.2 atg of Au ______ x

1. x = 0.2. 6.02 x 10²³

x = 1.204 x 10²³ gold atoms

question 3

If we compare equal masses of the following substances: NaCl, H₂O_2, HCl and H₂O. Which one has the greatest number of molecules?

See Answer

Correct answer: The water molecule.

The number of moles of each substance is: NaCl (58.5 g), H₂O₂ (34 g), HCl (36.5 g) and H₂O (18 g)

According to Avogadro's law, the number of molecules will be greater when the substance has a greater number of moles. To obtain the amount of moles, the following formula can be used:

No. mol = m/MM, where: m = mass of substance in grams, MM = molar mass

Thus, it can be concluded that among the substances above, the one with the lowest molar mass is H₂O (18g) and therefore has the largest number of molecules.

Done in another way, if we use 20 g as mass number, we will have:

• No. mol NaCl = 20 g/58.5 g/mol = 0.34 mol
• No. mol H₂O₂ = 20 g/34 g/mol = 0.59 mol
• No. mol HCl = 20 g/36.5 g/mol = 0.55 mol
• No. mol H₂O = 20 g/18 g/mol = 1.11 mol

Also read about Stoichiometry and Equation Balancing.