Calculate the mass of urea (NH2CONH2) required in making 2.5 kg of 0.25 molal aqueous solution.

Calculate the mass of urea (NH₂CONH₂) required in making 2.5 kg of 0.25 molal aqueous solution.

Solution

Given that

Mass of solution = 2.5 Kg

Molality = 0.25 m

Molar mass of urea (NH₂CONH₂) =14 + 2 × 1 + 12 + 16 + 14 + 2 × 1 = 60 g mol⁻¹

Let mass of urea (NH₂CONH₂) = w gram 

Number of moles of urea  = w / 60 = 0.01667 mol

Mass of urea in Kg  = w/1000  = 0.001w kg 

 

Mass  of solvent = total mass  - mass  of solute  = 2.5  - 0.001 w kg

Use the above formula we get 

Cross multiply and solve for  the value of w we get

 

w = 37 g of urea