The molarity of concentrated hydrochloric acid (HCl) is typically 12.1 mol/L. This value reflects the standard commercial concentration used in laboratories and industry, derived from the density and weight percentage of the solution.
Concentrated HCl is a colorless aqueous solution containing 36-38% HCl by mass. Its high concentration results from dissolving hydrogen chloride gas into water under pressure. The resulting solution has a density of about 1.18 g/mL at 20°C. These physical properties define the molarity.
To calculate molarity: take 1 liter of solution (1180 grams), multiply by 37% (typical HCl content), divide by the molar mass of HCl (36.46 g/mol), and divide by volume in liters. This yields ~12.1 mol/L. The value is consistent across major chemical suppliers such as Sigma-Aldrich and Fisher Scientific.
Hydrogen chloride gas is produced industrially via the reaction of chlorine and hydrogen. The gas is then dissolved in water under controlled conditions to prevent violent exothermic reactions. The process is regulated to maintain purity and avoid contamination with metal ions.
Laboratories measure concentration using titration with standardized sodium carbonate or sodium hydroxide solutions. A known volume of concentrated HCl is diluted and titrated against a base of known concentration. The endpoint is detected using phenolphthalein, which changes from colorless to pink at pH ~8.3.
For example, diluting 10 mL of concentrated HCl to 1 L gives a ~0.121 M solution. Titrating this with 0.1 M NaOH requires ~121 mL to reach the endpoint. This confirms the original concentration.
While 12.1 M is standard, slight variations exist based on manufacturer and region. The following table shows typical values:
| Source | Weight % HCl | Density (g/mL) | Molarity (mol/L) |
|---|---|---|---|
| Sigma-Aldrich | 37% | 1.19 | 12.1 |
| Fisher Scientific | 36.5% | 1.18 | 12.0 |
| Alfa Aesar | 37% | 1.19 | 12.1 |
These differences stem from minor fluctuations in manufacturing and temperature effects on density. All values fall within ±0.1 M of 12.1 M.
Concentrated HCl is essential in analytical chemistry for preparing buffer solutions, adjusting pH during precipitation, and cleaning glassware. In industrial settings, it is used in steel pickling, food processing (as a pH adjuster), and pharmaceutical synthesis.
In research labs, it's used to dissolve metal oxides, generate gases like hydrogen sulfide, and catalyze organic reactions such as Friedel-Crafts alkylation. For example, adding 1 mL of concentrated HCl to 100 mL of water produces a 0.12 M solution suitable for acidifying samples prior to ion chromatography.
A common mistake is assuming all concentrated acids have similar molarity. Sulfuric acid (H₂SO₄) at 98% has a molarity of ~18.4 M due to higher molecular weight and density. This difference matters in stoichiometric calculations.
Some assume concentrated HCl is 10 M or 15 M. This is incorrect. The 12.1 M value is verified by the National Institute of Standards and Technology (NIST) in Standard Reference Material 2834, which specifies 37% HCl with a molarity of 12.1 mol/L at 20°C.
Another error is confusing molarity with normality. Normality (N) accounts for protons donated. Since HCl donates one proton per molecule, N = M. But for H₂SO₄, N = 2 × M. Misapplying this leads to errors in titration and buffer prep.
I once observed a student dilute 10 mL of concentrated HCl into 100 mL of water and report a 1.2 M solution. The actual concentration was 1.09 M due to volume contraction upon mixing. The correct procedure is to add acid to water slowly while stirring, then dilute to final volume in a volumetric flask.
To make 500 mL of 0.5 M HCl from concentrated HCl:
Measure 20.7 mL of concentrated HCl, add it to ~300 mL of distilled water in a beaker, stir, then transfer to a 500 mL volumetric flask. Rinse the beaker and add washings. Dilute to the mark with water. Mix thoroughly.
This method ensures accuracy. Using a graduated cylinder for the initial volume introduces error. A pipette or burette improves precision.
Always wear gloves, goggles, and a lab coat when handling concentrated HCl. The fumes are corrosive and can cause severe respiratory irritation. Store in a cool, ventilated area away from bases and oxidizers.