A solution is prepared at 4.0 °C temperature by dissolving 134.5 g of potassium nitrate in 750 mL of water. what's the molarity and molality of the resulting solution if it has a volume of 745 mL

Answer 1

First, look up the density of water at 4.0ºC. I found it to be 1.000 g / ml. That makes it easy.

Molarity = moles solute / liter solution

moles solute = 134.5 g KNO3 x 1 mol KNO3 / 101.1 g = 1.330 mols

liters of solution = 745 ml x 1 L / 1000 ml = 0.745 L

Molarity (M) = 1.330 mol / 0.745 L = 1.786 M = 1.79 M (3 sig. figs.)

Molality = moles solute / kg solvent

moles solute = 1.330 mols (see above calculation)

kg of water = 750 ml x 1 g / ml = 750 g x 1 kg / 1000 g = 0.750 kg

molality (m) = 1.330 mol / 0.750 kg = 1.77 m (3 sig. figs.)