Question

Calculate the molality of Ca(OH)2 in a 1.50 M aqueous solution that has a density of 1.320 g/mL.​

Answer 1

Given:

molarity = 1.50 M

density of solution = 1.320 g/mL

solute: Ca(OH)₂

Required:

molality

Solution:

Step 1: Determine the number of moles of solute and the volume of solution.

1.50-M solution means that there are 1.50 moles of solute in 1 L of solution.

moles of solute = 1.50 mol

volume of solution = 1 L = 1000 mL

Step 2: Calculate the molar mass of solute.

molar mass of solute = (40.08 g/mol × 1) + (16.00 g/mol × 2) + (1.008 g/mol × 2)

molar mass of solute = 74.096 g/mol

Step 3: Calculate the mass of solute.

mass of solute = moles of solute × molar mass of solute

mass of solute = 1.50 mol × 74.096 g/mol

mass of solute = 111.144 g

Step 4: Calculate the mass of solution.

mass of solution = density of solution × volume of solution

mass of solution = 1.320 g/mL × 1000 mL

mass of solution = 1320 g

Step 5: Calculate the mass of solvent.

mass of solvent = mass of solution - mass of solute

mass of solvent = 1320 g - 111.144 g

mass of solvent = 1208.856 g = 1.208856 kg

Step 6: Calculate the molality of solution.

$\text{molality} = \frac{\text{moles of solute}}{\text{mass of solvent (kg)}}$

$\text{molality} = \frac{\text{1.50 mol}}{\text{1.208856 kg}}$

$\boxed{\text{molality = 1.24 m}}$