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I need to find flask weight. Instructions unclear to me.

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Suspicious_boat_5926:
Im not sure of how to proceed and Im kind of just stumbling along.

The class lab had us pump dry air into the flask for 5 minutes, then plug it and weigh it.

There is a dry air chart.

So far I have these steps:

1. Fill flask with dry air.
2. Weigh flask.
3. Find volume of flask.
4. Use volume of flask and dry air density to find the mass of air.
5. Subtract mass of air from air+flask+stopper to find flask weight.

The instructions dont say to weigh any of this stuff independently. I dont think I'll be able to correct for the weight of the plug (cork and small rubber stopper for cork hole).

6. Fill the flask with CO2.
7. Plug and weigh flask with CO2.
7a. Measure temp of flask atfter weighing CO2
8. Fill flask with water to brim, plug, wipe outside dry, weigh.
9. Use this to find the volume.

Instructions were not given to measure temp, but temp was measured for the CO2 after it cooled, so I suppose I just use room temp for the water.

I think I need to use the density of water table with temperature to find volume, however in all of this I still dont have the mass of the flask, so I cant exactly use water mass and flask mass to find volume.

Measured stuff:

Mass of flask with stopper, plug, filled with air. in grams. 131.4413
Mass of flask with stopper, plug, filled with CO2. in grams. 131.6014
Mass of flask with stopper, plug, filled with water. in grams. 396.64
Temp of flask 21.0 C
Baro pressure 755.18 torr
Absolute temp 294.K
Pressure 0.99366 atm.

The instructions for the lab discuss buoyancy as well. Is this part of how to discover the mass of the flask using only water stopped at the top of the vessel and weighing the mass of both flask and water together and using the density of water table for 21.0 C?

Further question, should I be limiting atm to significant digits when converting from torr? I already did here. Should I be limiting temp to significant digits from celcius? Round two has 21.4 C, so there are 4 actual digits in the Kelvin temp, but I rounded it to 294. again.

Hunter2:
Start with weight the flask with water. Then pure out the water in an other container and weight  the water or measure the milliliters. The difference gives the mass of the empty flask.

Ideal gas equation, in combination with  formula of density

pV = nRT , n = m/M and σ = m/V

m (total) = m(compound) + m(flask)

3 equations.

Combine all together.

Suspicious_boat_5926:
Thanks. I suppose I was supposed to know to do this in this lab. It makes sense. Pour into a measuring device for mL.

Without instructions saying to do that, I didn't think of it. Its too late to go back and do that now.

Is there any possible way to figure this out with calculations after the fact knowing only what I know about the mass of the water + flask and the atmospheric pressure + water density?

Borek:
If I understand the situation correctly you omitted point 3. during lab, yes?

Mass of of the flask with water minus mass of the flask filled with the air will give you mass of water, that can be used to convert to volume with density tables, perhaps as close to the real thing as possible in this case.


--- Quote from: Suspicious_boat_5926 on April 09, 2024, 12:14:40 AM ---Further question, should I be limiting atm to significant digits when converting from torr? I already did here.
--- End quote ---

Only when listing the number, but use full precision you have when plugging the number into next calculation steps.


--- Quote ---Should I be limiting temp to significant digits from celcius? Round two has 21.4 C, so there are 4 actual digits in the Kelvin temp, but I rounded it to 294. again.
--- End quote ---

273.15 is an exact number, and sigfigs rules for addition are different (the answer should have the same number of decimal places as the limiting term). I would list 273.15+21.4 as 294.5

Suspicious_boat_5926:
Thank you. I asked the prof today and it turns out the solution was to use the mass of the flask, with the dry air in it, with the stopper, and subtract the mass of water + flask + stopper from the former to obtain the mass of the water, and then use that to obtain the volume and the mass of the empty flask, and then use that to calculate the mass of the dry air in the flask, and then the mass of CO2 in the flask from that.

And it was close, like 43. gmol for CO2 by calculation at ~1.4% off.

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