Molarity of 37% (w/w) Hydrochloric Acid (HCl)

Overview

• Hydrochloric acid is a clear colorless aqueous solution of hydrogen chloride (HCl) that can be supplied as a 37% (w/w) hydrogen chloride solution.
• 37% (w/w) hydrochloric acid means that 100 g of hydrochloric acid contains 37 g of hydrogen chloride.
• The density of 37% (w/w) hydrochloric acid is 1.2 g/mL at 25 °C. This means 1 mL of hydrochloric acid has a mass of 1.2 g at 25°C.
• Molarity refers to the number of moles of the solute present in 1 liter of solution.
• The molar mass of a compound is the mass of one mole of it, expressed in grams per mole (g/mol), and is numerically equal to its molecular weight. For hydrogen chloride the molecular weight is 36.46, so one mole of HCl has a mass of 36.46 g.
• Precautions: Concentrated HCl is a very strong acid that will burn your skin instantly upon contact! Wear gloves, goggles, and lab coat.

Preparation

Step 1:

Calculate the volume of 100 grams of Hydrochloric acid.

Formula:

Density =  weight / volume or

Volume =  weight / density

Volume of 100 gram of Hydrochloric acid   =   100/1.2   =   83.33 ml

Note: 37% (w/w) Hydrochloric acid means that 100 grams of Hydrochloric acid contain 37 grams of Hydrogen chloride.

The volume of 100 grams of Hydrochloric acid is 83.33 ml. That means 37 grams of Hydrogen chloride is present in 83.33 ml of Hydrochloric acid.

Step 2:

Calculate how many grams of Hydrogen chloride is present in 1000 ml of Hydrochloric acid.

83.33 ml of Hydrochloric acid contains       =  37 g of Hydrogen chloride.

1 ml of Hydrochloric acid will contain    =  37/83.33 g of Hydrogen chloride.

1000 ml of Hydrochloric acid will contain =  1000 x 37/83.33   = 444.02 g of hydrogen chloride

1000 ml of Hydrochloric acid will contain 444.02 g of Hydrogen chloride.

Step 3:

Calculate the number of moles of Hydrogen chloride present in 444.02 g of Hydrogen chloride.

36.46 grams of Hydrogen chloride           =   1 mole

1 gram of Hydrogen chloride                    =   1/36.46 moles

444.02 grams of Hydrogen chloride   =    444.02 x 1/36.46   =   12.18 moles

Therefore, we can say that 1 liter of Hydrochloric acid contains 12.18 moles (rounded to 12.18 M) or in other words molarity of 37% (w/w) Hydrochloric acid is equal to 12.18 M.

Suppliers typically specify the density of 37% (w/w) HCl as 1.18–1.19 g/mL at 25 °C, which gives 11.97–12.08 M. The value obtained depends on the density figure used, so take it from the certificate of analysis for the lot in hand rather than a textbook.

References:

1. https://pubchem.ncbi.nlm.nih.gov/compound/Hydrochloric-acid