Help with a Chemistry Molality problem?
 

Working on some online chem HW... Me and several class mates have all worked this and the computer keeps marking it wrong... Want to check and make sure we aren't idiots before I go talk to the teacher...


Given info
An aqueous NaCl solution is made using 140g of NaCl diluted to a total solution volume of 1.15L

Calculate the molality of the solution. (Assume a density of 1.08 for the solution.)


I got 1.93m


Thoughts?


Thanks guys. SmileWavy

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I always prefer my molality above 3.2. No maybe that is my level of C2H5OH in frosty wheat pops.

hmm, I assume that the (Assume a density of 1.08 for the solution) is relevant. The normal way to calculate Molality is moles of solute / kilograms of solvent. So, with a molecular weight of 58.4 for NaCl, and assuming 25*C water, 1.15kg, then you've got 2.4 moles of salt / 1.15kg of water for 2.09 molality.

Glen "wheet" is not a compound I am familiar with but it doesn't sound satisfying.....

Ah, I see where the 1.08 comes in. The total solution volume is 1.15, so that's not how much water you started with, thats the volume of the water and salt, so you've got to figure out how much water there is based on the density. Yeah, figured it was important and not as basic as above.

I assume the 1.08 is g/ml, that means that 1.15l of water that weighs 1.08g/ml weighs 1242g, remove the 140 g of salt, and you've got 1102g of water which equals 1.102kg. Then 2.4moles of salt / 1.102kg of water gives 2.18 m.

What he said.

In times of stress, write out all your knowns *with units* then do dimensional analysis to see what you can cancel.

That got it... Guess I am just an idiot...



Oh and whoever came up with this stupid online chem... I've got some sign language