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
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.
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.
Thank you so much!
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.
To calculate the mass of the dry material needed to make an aqueous solution with a molality of 4 moles/kg solution, you need to follow these steps:
Step 1: Calculate the molar mass of Magnesium Sulfate (MgSO4).
The molar mass of MgSO4 is calculated by summing the atomic masses of the individual atoms:
Mg: 24.305 g/mol
S: 32.065 g/mol
O: 15.999 g/mol (there are 4 oxygen atoms in MgSO4)
So, the molar mass of MgSO4 is:
24.305 + 32.065 + (4 * 15.999) = 120.366 g/mol
Step 2: Calculate the moles of MgSO4 needed.
Since the molality is given as 4 moles/kg solution and the molality is defined as moles of solute per kilogram of solvent, you need 4 moles of MgSO4 for every 1 kilogram of solvent.
Step 3: Convert moles to grams.
To convert moles to grams, multiply the moles by the molar mass of MgSO4 calculated in Step 1.
For example, if you want to make a solution with 4 moles of MgSO4, you would calculate the mass as follows:
4 moles MgSO4 * 120.366 g/mol = 481.464 g
So, the mass of the dry material needed to make the solution is approximately 481.464 grams.
As for the mass of water needed, it will be 1 kilogram. The molality is the ratio of moles of solute to kilograms of solvent, so in this case, the mass of water (solvent) needed is 1 kilogram.
To summarize:
- Mass of dry material (MgSO4) needed: approximately 481.464 grams
- Mass of water needed: 1 kilogram