How to calculate normality? : 37% HCl solution
The Correct Answer and Explanation is:
How to Calculate Normality of 37% HCl Solution
Step-by-step Calculation:
- Given Data:
- Percentage purity of HCl = 37% w/w
- Density of HCl solution = 1.19 g/mL (at 25°C, standard value)
- Molar mass of HCl = 36.46 g/mol
- HCl is a monoprotic acid (releases 1 H⁺ per molecule)
- Assume volume of solution = 1 L (1000 mL)
- Mass of solution = Volume × Density = 1000 mL × 1.19 g/mL = 1190 g
- Mass of pure HCl = 37% of 1190 g = (37/100) × 1190 = 440.3 g
- Moles of HCl = Mass / Molar mass = 440.3 g / 36.46 g/mol ≈ 12.08 mol
- Since HCl is monoprotic, Normality (N) = Molarity × Basicity = 12.08 × 1 = 12.08 N
Explanation (300 words):
Normality (N) is a concentration unit used in acid-base and redox reactions. It reflects the gram equivalent of solute per liter of solution. For acids like hydrochloric acid (HCl), which completely dissociate and donate one proton (H⁺) per molecule, normality is numerically equal to molarity. However, when dealing with concentrated solutions (e.g., commercial HCl), the normality must be calculated using density and percentage composition rather than volume-based molarity directly.
To compute normality of a 37% HCl solution, begin with understanding the composition. A 37% solution by weight means there are 37 grams of HCl in every 100 grams of solution. When you know the density (1.19 g/mL), you can find the mass of 1 liter (1000 mL) of solution, which turns out to be 1190 grams. Applying the percentage, you determine that 440.3 grams of this mass is pure HCl.
Using HCl’s molar mass (36.46 g/mol), convert this mass into moles. Since HCl is monoprotic (1 mole = 1 equivalent), the number of moles is the same as the number of equivalents. Dividing this by the volume of the solution in liters gives the normality.
Thus, for a 37% HCl solution, the normality is approximately 12.08 N. This high value is why concentrated HCl is used with great caution in laboratories and industrial processes. It’s important to remember that the value might slightly vary depending on the exact temperature, as density changes with temperature.
