Question

The molarity of NaNO3 is1 M. The density of solution is 1.25g/ml. Calculate molality of solution. Molar mass of NaNO3= 85g

Answer 1

Given that, 1M solution of NaNO3,
By definition if Molarity we know that it is the no. of moles present in per litre of solution. => 1M molar solution has 1 mole of NaNO3 for 1000ml of solution.
Therefore, mass of solute is mass of 1 mole of NaNO3 = 23+14+48 = 85 gm.
volume of solution = 1000 ml as per definition.

Now,Given that density is 1.25 g/cm3
Therefore, mass = 1.25 * 1000 (m = density*volume)
=> mass = 1250 gm (solution mass)
Mass of solvent = mass of solution - mass of solute
= 1250 - 85= 1165gm.

Molarity is the no. of mole of solute per kg of solvent
=> m = (1/1165) * 1000 = 1000/1165 = 0.85 m (approx)


hope you understood it
Your teacher