I really need help with this molality question?

Question

The molality of lead nitrate in 0.726 M Pb(NO3)2 (density 1.202 g/mL) is ____________________.

Question 1 answers
0.604 m
0.873 m
0.755 m
1.32 m
1.23 m

If you could be explain how you got the answer please

Answer 1

hmmm im gonna guess 0.755m...

Answer 2

Lead nitrate is Pb(NO3)2.
Molarity means Moles/Liter.
Density=mass/volume. (in this case, g/mL)
So what I would do first is convert 1.202g/mL to g/L:
1.202 g/mL x (1000mL/L) = 1202 g/L Pb(NO3)2
That means that in one liter of lead nitrate, you have 1202 grams of it. Now we must convert grams to moles to find the molarity:
1202 g Pb(NO3)2 x (1 mol Pb(NO3)2/331.22 g) =


... Wait. Are you sure you have the question right? I think something's missing, because the question states that it's 0.726 MOLARITY Lead nitrate, and if so, then you're definitely missing something because the question makes no sense.
?

Ooh reading the post below me, I see that molality is not a type and it's different from molarity. :) So, um, ignore my post.

Answer 3

I'd try this:

Molality is moles per kg of solvent, little m. Molarity is moles per liter of solution, big M.

You have a .726 Molarity solution, or .726 moles per liter. You need to find out the weight of your solvent. Assume you have 1000 ml or 1L. Multiply this by the density, (1000ml*1.202g/ml) and this gets you 1202g, the weight of your solution.

Now if the solution is 1202g, and you subtract the weight of .726 mole of the lead nitrate, about 150 g,, you have the weight of your solvent, or 1052 g. So your molality is .726m/1.052kg or about .755m.

I think the trick here is that it's moles per kg of solvent, not total solution, so you need to get the weight of the solvent.

Answer 4

molality = 0.604

molality = moles solute / kg solution

molarity = moles solute / L solution
0.726 M = 0.726 moles / 1 L solution

However, this is not water, so the density of the solution must be used to determine the mass in kg of the solution:
1 L = 1.202 kg (1.202 g/ml = 1202 g/L = 1.202 kg / L)

Since molality = moles / kg you must divided the number of moles by 1.202 kg to calculate the molality.

0.726 moles / 1.202 kg = 0.604 molal solution.