Calculate the molarity and molality.

Question

An intraven solution is analyzed to confirm its concentration. A 250 mL sample with a mass of 251.60 g is evaporated to dryness. The solid sodium carbonate residue has a mass of 2.27 g. Calculate the molarity of the solution and the molalitiy  

Answer 1

Molarity = moles solute/liter solution

Molality = moles solute/kg solvent

 

Let's first find moles of solute:

2.27 g Na2CO3 x 1 moles/106 g = 0.0214 moles solute

 

Next, we can find the mass of solvent:

251.60 g solution - 2.27 g solute = 249.33 g of solvent = 0.24933 kg solvent

 

The volume of solution is given as 250 ml = 0.250 liters.

So, 

MOLARITY = moles solute/liter solution = 0.0214 moles/0.250 liters = 0.0856 M (3 significant figures)

MOLALITY = moles solute/kg solvent = 0.0214 moles/0.24933 kg = 0.0858 m (3 significant figures)