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Topic: apparant mass of KNO3, weighed in air?  (Read 7241 times)

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apparant mass of KNO3, weighed in air?
« on: September 20, 2006, 12:21:52 AM »
this problem is on my homework and i absolutely don't know how to go about solving it at all! can anyone please help me out! iit goes:

You want to prepare 500ml of 1.000 M KNO3 at 20 degrees C but the lab and water temperature is 24 degrees C at the time of preparation.  How many grams of KNO3 (density= 2.109 g/ml) should be dissolved in a volume of 500.00mL at 24 degrees C to give a concentration of 1.000 M at 20 degrees celcius?  What apparant mass of KNO3, weighed in air is required?

the answer to this problem is True mass: 50.506 g, mass in air = 50.484g

i just don't know how they got these answers!!! please *delete me*
« Last Edit: September 20, 2006, 03:56:23 AM by Mitch »

Offline Borek

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Re: Wondering if ANYone can solve this problem? really need help
« Reply #1 on: September 20, 2006, 02:55:37 AM »
I suppose I know what the question is TRYING to ask about, but it is wrong in many aspects.

You are probably expected to take Archimedes law into account. Your solid is submerged in the air and its apparent mass is smaller by the weight of the displaced air. You know the mass of the solid (do you? - read on), you have its density, that's enough to calculate solid volume. Use air density at 24 deg C to calculate mass of displaced air - tha's the difference bewteen real and apparent mass of the solid weighted. That's what you are IMHO expected to do.

<rant mode on>

But then... Solid should be not dissolved in 500.00 mL, it should be FILLED UP to 500.00 mL. It also lloks like the water has a temperature of 24 deg C, so the solution will contract on cooling to 20 deg - which will change the concentration. Do you have to take that into account? Probably not, but if you are asked to be so precise when weighting, than why are you to ignore other sources of errors? And finally answer given is - once again IMHO - incorrect. 50.506g is not half a mole of KNO3, as it is calculated using wrong molar mass (should be 101.1032g). Where is the sense of asking imprecise question about superprecise weighting?

</rant mode on>
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Offline mike

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Re: Wondering if ANYone can solve this problem? really need help
« Reply #2 on: September 20, 2006, 03:01:21 AM »
Can you write the question as it is originally worded?
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