Calculate mass in solution

[giorgiog01]

Im stuck on this practical question below because i believe i don't have enough information to answer it. If anyone could point me in the right direction it would really help:

-Calculate the mass of a compound with a molar mass 392 in 100cm³ of solution


All i know was to:

accurately weigh 0.7g of this compound and transfer it into 100cm³ graduated flask containing 80cm³ distilled water. shake to dissolve and then make the solution up to 100cm³ with distilled water.



As im being asked to calculate the mass of the compound i cant use the 0.7g (as i'm supposed to be calculating this), i cant use mass=conc X volume as i don't know the concentration. so i can only think of using mass= number moles X Molar mass but i dont know how many moles there are.

Only other think i can think of is using the density/volume of water i used but that's presuming the volume of the compound in the 100cm³ is negligible which i havent been told to presume.
surely i need more information such as concentration or total mass of compound and solution?

Thanks to anyone who helps!

[Iota]

You're right, "Calculate the mass of a compound with a molar mass 392 in 100cm³ of solution" makes no sense here.

 

You already know its mass, which is what you added (0.7g). Therefore there's no calculation needed to work it out.

 

 

On the other hand, with the information you have available, i.e. RMM, mass, and liquid volume- you can calculate the concentration, which you don't know, using the mass which you do know.

Edited December 7, 2013 by Iota

[giorgiog01]

yeah i agree. just completely confused as the next question is:
-compare your calculated mass with the weighed quantity.

must be a missprint/i'm missing some information. Thank you for confirming