Question

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

Answer 1

First thing to note, kg and liters are inter convertible,wont make any difference.

0.25 molar solution means, there is 0.25 moles of urea in one liter,so to find in 2.5 liters.

0.25×2.5=0.625 moles of urea is required.


No of moles=$\frac{Mass}{Molar mass}$

We know,

no of moles=0.625
Molar mass of urea= 14+2×1+12+16+14+2×1=60 g/mol

Mass=No of moles×Molar mass
        =0.625×60
        =37.5 g

Answer 2

Hey !!

Mass of required aqueous solution = 2.5 kg = 2500 g

0.25 molal aqueous solution of urea it means that 0.25 mole of urea is dissolved in 1000 grams of water.

Mass of water = 1000 g

Mass of urea = 0.25 mol

Molar mass of urea (H₂NCONH₂) = 4 × 1 + 2 × 14 + 1 × 12 + 1 × 16 = 60 g mol⁻¹

        NOTE :- Mass of urea = Number of moles of urea × Molar mass of urea  

     Mass of 0.25 moles of urea = 0.25 mol ×× 60 h mol⁻¹ = 15 g

Mass of solution = 1000 g + 15 g = 1015 g

1015 g of aqueous solution contains urea = 15 g

∴ 2500 g of aqueous solution will require urea  

         = 15 g / 1015 kg × 2500 g  

            = 36.95 g

HOPE IT HELPS YOU !!