To calculate the molality of a solution, we first need to understand the relationship between density and molarity.

Density: Density is the mass of a substance per unit volume. It is expressed as grams per milliliter (g/ml) or grams per cubic centimeter (g/cm³). In this case, the density of the NaCl solution is given as 1.25 g/ml.

Molarity: Molarity is the number of moles of solute per liter of solution. It is expressed as moles per liter (mol/L) or molar (M). The molarity of a solution can be calculated using the formula:

Molarity (M) = (moles of solute) / (volume of solution in liters)

Molality: Molality is the number of moles of solute per kilogram of solvent. It is expressed as moles per kilogram (mol/kg). The molality of a solution can be calculated using the formula:

Molality (m) = (moles of solute) / (mass of solvent in kg)



To calculate the molality of the 3M NaCl solution, we need to convert the given molarity into moles and the given density into mass of solvent.

We know that the molarity of the solution is 3M, which means there are 3 moles of NaCl in every liter of solution.

To convert the molarity into moles, we can use the formula:

moles of solute = molarity x volume of solution (in liters)
moles of solute = 3M x 1L = 3 moles

Now, we need to determine the mass of the solvent (water) in the solution. We can use the given density to calculate the mass of 1 ml of the solution.

mass of 1 ml of solution = density x volume
mass of 1 ml of solution = 1.25 g/ml x 1 ml = 1.25 grams

Since the density is given in grams per milliliter, the mass of 1 ml is equal to its volume in grams.

To convert the mass of the solvent into kilograms, we divide the mass by 1000 (1 kg = 1000 g).

mass of solvent = mass of 1 ml / 1000 = 1.25 g / 1000 = 0.00125 kg

Finally, we can calculate the molality using the formula:

Molality (m) = moles of solute / mass of solvent in kg
Molality (m) = 3 moles / 0.00125 kg = 2400 mol/kg

Therefore, the molality of the 3M NaCl solution is 2400 mol/kg.