[Gobo]

Find the mass of urea (CH4N2O) needed to prepare 51.8g of a solution in water in which the mole fraction of urea is 7.55×10−2.

Is this a valid assumption?
The molar mass of the solution is MM(H2O) + MM(NH4N2O)

So then, MM(sol'n) = (18.02 + 60.062)g/mol sol'n = 78.082g/mol sol'n
-------------------------------------------------------------------------------------------------------------------

If the mole fraction of urea is 0.0755, then the mole fraction is water is 0.9245.
I would like a hint, and know if my assumption will work.

[sjb]

Find the mass of urea (CH4N2O) needed to prepare 51.8g of a solution in water in which the mole fraction of urea is 7.55×10−2.

Is this a valid assumption?
The molar mass of the solution is MM(H2O) + MM(NH4N2O)

So then, MM(sol'n) = (18.02 + 60.062)g/mol sol'n = 78.082g/mol sol'n
-------------------------------------------------------------------------------------------------------------------

If the mole fraction of urea is 0.0755, then the mole fraction is water is 0.9245.
I would like a hint, and know if my assumption will work.

First stab... not really, no.

What would the mass of 0.9245 mol of water be, and of 0.0755 mol of urea? What is the total mass? So what factor would you need to scale by to get a total solution of 51.8g...

[Gobo]

Find the mass of urea (CH4N2O) needed to prepare 51.8g of a solution in water in which the mole fraction of urea is 7.55×10−2.

Is this a valid assumption?
The molar mass of the solution is MM(H2O) + MM(NH4N2O)

So then, MM(sol'n) = (18.02 + 60.062)g/mol sol'n = 78.082g/mol sol'n
-------------------------------------------------------------------------------------------------------------------

If the mole fraction of urea is 0.0755, then the mole fraction is water is 0.9245.
I would like a hint, and know if my assumption will work.

First stab... not really, no.

What would the mass of 0.9245 mol of water be, and of 0.0755 mol of urea? What is the total mass? So what factor would you need to scale by to get a total solution of 51.8g...

OH! So we could say per 1 mole of solution, we have 0.9245mol of H2O, and 0.0755 mol of urea, just like how I did above?

[Borek]

The molar mass of the solution is MM(H2O) + MM(NH4N2O)

No such thing as a 'molar mass of the solution'. Solution is a mixture, molar mass is a property of a single substance.

[Gobo]

The molar mass of the solution is MM(H2O) + MM(NH4N2O)

No such thing as a 'molar mass of the solution'. Solution is a mixture, molar mass is a property of a single substance.

Thank you. That clears any subsequent attempts for that approach.

OH! So we could say per 1 mole of solution, we have 0.9245mol of H2O, and 0.0755 mol of urea, just like how I did above?