Solution:

Given,
Mass of NH3 = 0.85 g
Volume of solution = 100 mL = 0.1 L
Density of solution = 1.2 g/mL

We need to find the molality of the solution.

Formula:

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

1. Calculation of number of moles of solute

The molar mass of NH3 = 14 + 3(1) = 17 g/mol

Number of moles of NH3 = Mass of NH3 / Molar mass of NH3
Number of moles of NH3 = 0.85 g / 17 g/mol
Number of moles of NH3 = 0.05 mol

2. Calculation of mass of solvent

Mass of solvent = Volume of solution x Density of solution
Mass of solvent = 0.1 L x 1.2 g/mL
Mass of solvent = 0.12 kg

3. Calculation of molality

Molality (m) = Number of moles of solute / Mass of solvent in kg
Molality (m) = 0.05 mol / 0.12 kg
Molality (m) = 0.42 mol/kg

Therefore, the molality of the solution is 0.42 mol/kg.

Explanation:

Molality is a measure of concentration in a solution. It is defined as the number of moles of solute dissolved in 1 kg of solvent.

In this problem, we are given the mass of NH3 and the volume of solution. We need to calculate the molality of the solution.

First, we calculate the number of moles of NH3 using its molar mass. Then, we calculate the mass of the solvent using the density of the solution. Finally, we use the formula of molality to calculate the molality of the solution.

It is important to note that molality does not depend on the temperature but it depends on the mass of the solvent. It is a more accurate measure of concentration compared to molarity, which depends on the volume of the solution and can change with temperature and pressure.