Question

Calculate the molality of the following solution containing 182.5g of HCl in 800g of H20
When do you use molality rather than molarity? Explain why?
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Answer 1

Given,

=>182.5 g of HCl is present in 800 g of H2O.

=>182.5 g of HCl is present in 800/1000 kg => 0.8 kg of H2O.

We know that,

Molar mass of HCl = 1 + 35.5 => 36.5 grams per mole.

Molality = (moles of solute) / (kilograms of solvent)

To find,

Number of moles in 182.5 grams of HCl = 182.5/36.5 => 5 moles of HCl.

Molality = 5 moles of HCl/0.8 kilograms of water =>50/8=>25/4 =>6.25

Hence, the molality of the solution is 6.25 mol kg-1 .

Question: When do you use molality rather than molarity? Explain why?

Answer: When doing calculations involving vapour pressure and changes in temperatures molality is generally used and not molarity.

This is because molality is moles of solute per kilograms of solvent, the mass of the solvent remains constant and does not change even with changes in temperature, but, the volume of a solution varies slightly with increase or decrease in temperature, hence in such cases molarity cannot be used and molality is preferred for accurate calculation.

In titrations and other analysis at fixed temperature and pressure conditions, molarity and normality can easily be put to use.