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Calculate the mass of urea `("NH"_(2)"CONH"_(2))` required in making 2.5 kg of 0.25 molar aqueous solution. We know that molarity (m) `=("Moles of solute")/("Mass of solvent in kg")` and moles of soute `=("Mass of solute")/("Molar mass of solute")` So, find the molar mass of solute by adding atomic masses of different element present in it and mass by using the formula, Molality `=("Mass of solute/molar mass of solute")/("Mass of solvent in kg")`

Answer» Molality of the solution `=0.25" m"=0.25" mol kg"^(-1)`
Molar mass of urea `("NH"_(2)"CONH"_(2))=(14xx2)+(1xx4)+12+16`
`=60" g mol"^(-1)`
Mass of solvent (water) = 2.5 kg
Molality `=("Mass of urea/molar mass of urea")/("Mass of water in kg")`
`(0.25" mol kg"^(-1))=("Mass of urea")/((60" g mol"^(-1))xx(2.5" kg"))`
Mass of urea `=(0.25" mol"^(-1))xx(60" g mol"^(-1))xx(2.5" kg")=37.5" g"`


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