|
|
|
|
| Author |
|
|
Cogito Ergo Sum
|
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. 
|
||
02-11-2010, 06:27 PM
|
|
|
Registered
|
|||
|
02-11-2010, 06:35 PM
|
|
|
Get off my lawn!
|
I always prefer my molality above 3.2. No maybe that is my level of C2H5OH in frosty wheat pops.
__________________
Glen 49 Year member of the Porsche Club of America 1985 911 Carrera; 2017 Macan 1986 El Camino with Fuel Injected 350 Crate Engine My Motto: I will never be too old to have a happy childhood! Last edited by GH85Carrera; 02-12-2010 at 05:16 AM.. |
||
|
02-11-2010, 06:39 PM
|
|
|
Back in the saddle again
Join Date: Oct 2001
Location: Central TX west of Houston
Posts: 55,938
|
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.
__________________
Steve '08 Boxster RS60 Spyder #0099/1960 - never named a car before, but this is Charlotte. '88 targa  SOLD 2004 - gone but not forgotten
|
||
|
02-11-2010, 06:39 PM
|
|
|
Cogito Ergo Sum
|
Glen "wheet" is not a compound I am familiar with but it doesn't sound satisfying.....
|
||
|
02-11-2010, 06:40 PM
|
|
|
Back in the saddle again
Join Date: Oct 2001
Location: Central TX west of Houston
Posts: 55,938
|
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.
__________________
Steve '08 Boxster RS60 Spyder #0099/1960 - never named a car before, but this is Charlotte. '88 targa SOLD 2004 - gone but not forgotten
|
||
|
02-11-2010, 06:50 PM
|
|
|
|
Back in the saddle again
Join Date: Oct 2001
Location: Central TX west of Houston
Posts: 55,938
|
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.
__________________
Steve '08 Boxster RS60 Spyder #0099/1960 - never named a car before, but this is Charlotte. '88 targa SOLD 2004 - gone but not forgotten
|
||
|
02-11-2010, 06:54 PM
|
|
|
Registered
|
Quote:
In times of stress, write out all your knowns *with units* then do dimensional analysis to see what you can cancel. |
||
|
02-11-2010, 07:23 PM
|
|
|
Cogito Ergo Sum
|
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 |
||
|
02-11-2010, 09:34 PM
|
|
1986 Porsche 944 2.5l
1986 Porsche 944 2.5L N/A
Carrera
1979 Porsche 911SC 3.0
