HOW TO DETERMINE THE MOLAR MASS OF HYDROCHLORIC ACID? (WITH CALCULATION EXAMPLE)

In this article, we will explain how to determine the molar mass of hydrochloric acid.

If you are looking for a quick answer, the molar mass of hydrochloric acid M = 36.453 g/mol. Below you will also find a detailed calculation example.

A label on a bottle with a concentrated hydrochloric acid. Image credit: maticulous via Flickr, CC BY 2.0

Let’s first start with some basic information about hydrochloric acid.

What is hydrochloric acid?

As the name of this chemical compound says, it is an acid and a very strong one. Due to its corrosive action, it belongs to a list of hazardous substances because it can cause severe burns.

However, despite being particularly harmful to the human body, it has many beneficial industrial uses.

For example, hydrochloric acid is used to produce chloride compounds, agricultural fertilizers, and dyes, refine ores and remove boiler scales.

In chemistry, hydrochloric acid has a molecular formula of HCl. This means its molecule consists of one hydrogen atom and one chlorine atom.

What is molar mass?

All substances take up a certain volume and have mass. In our everyday life, we can simply measure these parameters using rulers, scales, or other standard instruments.

However, all bulk materials are made of small particles – atoms and molecules. When performing chemical experiments it is important to know the precise numbers of these particles. But how do we know how many particles are there in a sample? Can we accurately measure something as small as a molecule?

Instead of weighing or counting molecules, scientists use the so-called molar mass of a chemical compound. While every molecule has its own mass which is called molecular mass, the molar mass is calculated as an average over many particles.

How many particles exactly? Definition of the molar mass

The concept of molar mass was defined as a separate measure of the amount of substance in 1971 by the International System of Units (SI).

Until 2019, the mole – unit of molar mass – was defined as the amount of substance containing as many particles as there are atoms in 12 grams of the carbon-12 isotope.

Since 2019, this definition was redefined: now a mole unit refers to the amount of a substance containing exactly 6.02214076×1023 particles. The molar mass of any compound is measured in g/mol (grams per mole) and shows what is the mass of this substance in grams when it contains exactly this number of molecules.

Substance amounts are measured in moles when scientists need to determine how much of reagents any particular chemical reaction would require.

Finding the molar mass of hydrochloric acid – calculation example

The molar mass is the sum of the total mass in grams of all the atoms that make up a single mole of a specific molecule. This means that the molar mass of hydrochloric acid can be determined as the sum of the molar masses of hydrogen (H) and chlorine (Cl) atoms that make up these HCl molecules.

How do we find these two molar mass values?

Step 1: Finding relative atomic masses of hydrogen and chlorine atoms

Relative atomic masses of individual chemical elements can be found using the periodic table of elements.

This information is written underneath the symbol and the name of the element. Please note that these numbers are not whole values but also have decimal parts.

• The relative atomic mass of hydrogen is 1.008
• The relative atomic mass of chlorine is 35.453

NOTE: The exact relative atomic mass of hydrogen is 1.00797 – for simplicity, we round this value to the third decimal place.

Step 2: Finding molar masses of chemical elements that make up the HCl molecule

In this step, we simply multiply the relative atomic mass by the molar mass constant, which is defined as 1 gram per mole.

This means that the molar mass of an atom numerically will be equal to its relative atomic mass, just units will be g/mol, not daltons (i.e. unified relative atomic mass units).

• The molar mass of hydrogen is 1.008 g/mol.
• The molar mass of chlorine is 35.453 g/mol.

Step 3: Calculate the molar mass of hydrochloric acid (HCl)

The molar mass is usually denoted as M. For hydrochloric acid, M would be calculated as a sum of the molar masses of hydrogen and chlorine atoms:

M = 1.008 (g/mol) + 35.453 (g/mol) = 36.461 g/mol.

This means that the molar mass of hydrochloric acid is M = 36.461 g/mol.

Summary

Always remember that hydrochloric acid is a very strong acid and in practice should be handled with the utmost care. Its molecule consists of one hydrogen (H) atom and one chlorine (Cl) atom and therefore is denoted as HCl. To find the molar mass of hydrochloric acid, we need to sum up the molar masses of hydrogen and chlorine. The molar mass of hydrochloric acid is M = 36.453 g/mol.