The molarity of a typical concentrated hydrochloric acid (HCl) solution is approximately 12.23 M. This value represents the concentration of pure hydrogen chloride dissolved in the solution, a crucial parameter for various chemical applications.
Understanding Molarity
Molarity (M) is a standard unit of concentration in chemistry, defined as the number of moles of solute per liter of solution. It's a fundamental concept for understanding how much of a substance is dissolved in a given volume, directly impacting chemical reactions and laboratory procedures.
Calculating the Molarity of Concentrated HCl
To determine the molarity of concentrated HCl, we consider its common characteristics: its density and its percentage by mass. Concentrated hydrochloric acid is typically sold at a high purity, often around 37% by mass, and possesses a density of approximately 1.19 kg/L.
Based on these characteristics, a specific amount of pure HCl can be found in one liter of solution:
- Mass of HCl per Liter: For a concentrated HCl solution, one liter is found to contain approximately 446.25 grams of pure hydrogen chloride.
- Molecular Weight of HCl: The molecular weight of HCl is approximately 36.5 grams per mole (g/mol).
- Moles of HCl: To determine the number of moles of HCl in one liter, we divide the mass of HCl by its molecular weight:
$$ \text{Moles of HCl} = \frac{\text{Mass of HCl}}{\text{Molecular Weight of HCl}} = \frac{446.25 \text{ g}}{36.5 \text{ g/mol}} = 12.23 \text{ moles} $$ - Molarity: Since 12.23 moles of HCl are present in one liter of solution, the molarity of this concentrated hydrochloric acid is 12.23 M.
Key Characteristics of Concentrated HCl (Approximate Values)
Understanding the approximate properties of concentrated HCl is essential for accurate laboratory work and safety.
| Property | Value |
|---|---|
| Molarity | 12.23 M |
| Percentage by Mass (w/w) | ~37% |
| Density | ~1.19 kg/L (or 1.19 g/mL) |
| Molecular Weight of HCl | 36.5 g/mol |
| Mass of HCl per Liter | ~446.25 g |
Practical Insights and Safety Considerations
The high molarity of concentrated HCl underscores several important aspects:
- Dilution: Concentrated HCl is frequently diluted to prepare solutions of lower molarity for various experimental needs. Always add acid slowly to water, not water to acid, to manage the exothermic heat generated during dilution.
- Chemical Reactions: The high concentration means a greater number of reactive HCl molecules per unit volume, leading to more vigorous reactions.
- Safety: Due to its high acidity and corrosive nature, handling concentrated HCl requires strict safety protocols, including:
- Wearing appropriate personal protective equipment (PPE) such as safety goggles, gloves, and a lab coat.
- Working in a well-ventilated area or under a fume hood to avoid inhaling corrosive fumes.
- Having immediate access to an eyewash station and safety shower in case of skin or eye contact.
- Storing in a dedicated acid cabinet, away from bases and oxidizing agents.
Variability in HCl Molarity
It's important to note that while 12.23 M is a common and often quoted molarity for concentrated HCl (typically 37% by mass), the exact concentration can vary slightly between manufacturers or batches. Always consult the specific label on the reagent bottle for precise information regarding its percentage by mass and density, which can then be used to calculate its exact molarity if needed.
For general laboratory purposes, however, 12.23 M serves as a reliable approximation for concentrated hydrochloric acid.
