Chemical Calculation: Molarity and Molality Calculation

How do we calculate the molarity and molality of a solution?

Given the following data: 50g of water dissolves 10g of NaCl with a density of 1.08 g/cm3.

Answer:

The correct option is b. 3.06 M and 3.4 m

Our solution consists of:

Solvent: NaCl (10g)

Solvent: water (50g)

First, we convert the mass of solute to moles:

10g * 1mol / 58.45g = 0.171 moles

Next, we convert the mass of solvent from grams to kilograms:

50g * 1kg / 1000g = 0.05kg

We calculate the molality of the solution:

m = moles / kg = 0.171 mol / 0.05 kg = 3.42 m

To calculate the molarity (M), we need the volume of the solution. We first add the mass of the solute and solvent to get the total mass of the solution:

50g + 10g = 60g of solution

Using the density formula (d = m / v), we can find the volume (v) of the solution:

60g / 1.08g/cm³ = 55.5 cm³

Converting the solution volume to milliliters (mL): 55.5 mL

Converting moles to millimoles and calculating the molarity:

0.171 mol * 1000 mmol / mol = 171 mmol

M = 171 mmol / 55.5 mL = 3.08 M