Question

How many moles of urea 60g/mol must be dissolved in 76.1g of water to give a 3.5 molal solution?

Answer 1

Molality is just the number of mols of the thing you're dissolving divided by the mass of what you're dissolving it in (in kg). So you know you've only got .0761 kg of water and you want to make it to 3.5 molal... just set up and solve this equation\[\frac{ n_{urea} }{ .0761 } = 3.5\]
just solve for n, the number of moles of urea. The molar mass of urea is extra information in this case.