Question

molarity of H2SO4 solution is 18M. If density of solution is 1.8g/cm3 . Calculate molality of the solution.

Answer 1

molality = mol H2SO4/kg solvent
molarity = mol H2SO4/L solution

Using density,
Mass of the solution = 1.8g/ml * 1000 ml = 1800g

Molecular weight of H2SO4 = 98g

Mass of solute (H2SO4) = 18 mol * 98 g = 1764 g

Mass of solvent = mass of solution - mass of solute
= 1800 g - 1764 g
= 36 g = 0.0036 kg

Molality = mol. H2SO4/ kg of solvent
= 18 mol/0.0036kg
= 500 mol/kg

Answer 2

What is Molality ?

- Molality ( m ) is expressed as moles of solute dissolved in hundred grams that is one kg of solvent it is denoted by m.

- It is calculate by : m = moles of solute / kg of solvent.

For example :- Molality of a solution of NaOH is 1m. This means that 1 mole of solute ( here:- NaOH ) is dissolved in 1 L of the solvent ( here :- water ).

And ,Molality is no. of moles of solute dissolved in litres of solution.

Given,

Molarity of H2SO4 solution is 18M and density of the solution is 1.8g/cm^3.

To find,

Molality of this solution.

Solution :

18M means that there are 18 moles of H2SO4 in 1 L of solvent. i.e 1000 ml

Now,

Mass of this solution = 1.8 g/ml × 1000 ml = 1800 g

Mass of solute = 18 × molar mass of H2SO4 = 18 × 98 = 1764 g

Mass of solvent = mass of solution - mass of solute = (1800 -1764) g = 36g = 0.036kg

Molality = (18/0.036)m = 500 m ( Final Answer )