Question

chemistry
posted by Naomi today at 8:03pm.

The material Magnesium Sulfate What mass of dry material would be needed to make an aqueous solution with a molality of 4 moles/kg solution ? What mass of water is needed?
4 (24.305+ 32.065+ (4 x 15.999)) =481.5 ?Not sure where to go from here or if the problem is complete.

chemistry - DrBob222 today at 9:11pm
Let me point out that your definition of molality is not right. The definition is m = #mols/kg SOLVENT and not kg SOLUTIION. So you have te mols calculated, place that is 1 kg solvent and you have it.

chemistry - Please check again still have questions (Confused!!) today at 9:37pm
So just divide it. The mass of the dry material is 481.5 g or kg???? I'm sorry I'm still confused about this. The mass of the water is 1 kg right??

chemistry - Please check again still have questions (Confused!!) today at 9:44pm
OR would the mass of the dry material be 481.5 M since we are dealing with molarity

Answer 1

Molality = (number of moles)/(Mass of solvent)

In this case, the number of moles (n) are of Magnesium Sulfate, and the solvent is water.

Now, we want a solution which is 4 molal, that means, it has 4 moles in every kg of water.

Now, if we take a solution which has 1 kg of water, it would need to have 4 moles of MgSO4 to have a molarity of 4 moles/kg

Required mass of MgSO4 = (number of moles)*(molecular mass)
= 4 * 120.66g
= 481.5 g

This amount is in grams, because Molar Mass is always taken in grams.

The mass of the material is 481.5 grams, and it is in a solvent (water) of mass 1 kg.

Answer 2

Addition:

"OR would the mass of the dry material be 481.5 M since we are dealing with molarity"

Take note that your question is asking for molality, and not molarity. The symbol for molality is m, not M.

Also, mass of a substance will always be in units of mass, such as g or kg. 'M', that is, 'Molar units' will never be used for mass.

Answer 3

Thank you so much!

Answer 4

To clarify what I wrote, you had the mass MgSO4 calculated correctly as 481.5 g. All you need to do is to place that in 1 kg watr and you have the 4 m solution.

And make sure you're not ovrlooking what both Arora and I have pointed out. The problem says MOLAL and not MOLAR. The definition you posted was for a M solutioin and not a m solution.