Question

Calculate the mass of urea required in making 2.5 kg of 0.25 molal aqueous solution

Answer 1

Concentration you need = 0.25 moles per litre

Volume = 2.5 kg = 2.5 litres

So if you need 0.25 moles for every litre you need 2.5 times that for 2.5 litres.

2.5 x 0.25 = 0.625 moles

So now you need to calculate to fine what 0.625 moles of Urea is.


1 mole of Urea NH2CONH2 =

2 x N = 2 x 14 = 28g
4 x H = 4 x 1 = 4g
1 x C = 1 x 12 = 12g
1 x O = 1 x 16 = 16g

1 mole of Urea = 28 + 4 + 12 + 16 = 60g


Number of moles needed = 0.625
Mass of 1 mole = 60g

mass of 0.625 moles = mass of 1 mole x 0.625

= 60 x 0.625 = 37.5g

Answer 2

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