Question

Calculate molality of 20 w/wsolution of acetic acid solution

Answer 1

Molecular weight of sulphuric acid

=98g/mol

There's 20g acid in 100g of water.

Thus actual mass of water present is 80g .

Molality means number of solute particles present per kg of solvent. Thus,80g of solvent(water) has 20g acid⇒1g of solvent has 2080g acid

⇒1000g(

=1kg) of solvent has 2080×1000=250g of acid.

Now,98g of acid means 1 mole of acid.

⇒1g

of acid means 198 moles of acid.

⇒250g of acid means 198×250=2.55 moles of acid.

Thus, molality of the solution is 2.55 molal because 2.55 moles of acid are present in 1000g of water

Or going by the conventional method, and applying the formula,

Molality =given massmolar mass×100g mass of solvent in g

=2098×100080=2.55 mol/kg