Question

2m urea solution is diluted from 2kg to 5kg what is the molality of diluted solution

Answer 1

Molar mass CO(NH2)2 = 60.06g/mol  

I think that a good place to start is to make a 2m CO(NH2)2 solution:  

A 2.0m solution contains 2 mol CO(NH2)2 dissolved in 1.0kg water.  

Mass of 2 mol CO(NH2)2 = 60.06*2 = 120.12g dissolved in 1.0kg = 1,000g water  

Total mass of 2.0m solution = 1,120.12g  

2kg of this solution will contain :  

Mass CO(NH2)2 = 2000/1120.12*120.12 = 214.48g CO(NH2)2  

The 2.0kg of solution contains :  

214.48g CO(NH2)2  

1,785.52g H2O  

You now increase mass of solution to 5.0kg – you add 3000g water  

Solution now is  

214.48g CO(NH2)2  

4,785.52g H2O  

Mass of CO(NH2)2 in 1000g H2O = 1000/4785.52*214.48 = 44.82 g CO(NH2)2  

Therefore 44.82g CO(NH2)2 dissolved in 1.0kg water.  

Moles of CO(NH2)2 = 44.82/60.06 = 0.746 moles CO(NH2)2 per kg water  

Molality of solution = 0.75m