Concentrated Nitric Acid Solution: Calculate the Molality

What is the molality of nitric acid in a concentrated solution of nitric acid (68.0% HNO₃ by mass)?

Can you determine the molality of a 68.0% HNO₃ solution?

Answer:

The molality of a 68.0% HNO₃ solution can be calculated by determining the moles of nitric acid and dividing by the kilograms of water. The solution has a molality of 33.7 mol/kg or 33.7m.

Explanation: The molality of a solution is defined as the moles of solute per kilogram of solvent. Here, the solute is nitric acid (HNO₃) and the solvent is water.

In order to calculate the molality of the solution, we'll first determine the mass of nitric acid in the solution. As the solution is 68.0% HNO₃ by mass, it means that there are 68.0g of HNO₃ per 100g of solution.

Thus, the mass of HNO₃ is 68.0g and the mass of water in the solution is (100g - 68.0g = 32g). We convert this to kilograms, giving us 0.032kg.

The molar mass of HNO₃ is approximately 63.01 g/mol. Therefore, we find the moles of HNO₃ by dividing the mass by the molar mass, or 68g / 63.01g/mol = 1.078mol.

Finally, molality is moles of solute per kilogram of solvent. So, dividing the moles of HNO₃ by the kilograms of water gives the molality of the solution. 1.078mol / 0.032kg = 33.7 mol/kg or 33.7m.