[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.

Further question, should I be limiting atm to significant digits when converting from torr? I already did here.

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

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.

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.

[Hunter2]

Strange

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,

This is mass(Flask,stopper  + water) - (Flask,stopper + air) = mass of water - mass of air??
This would work if the flask has nothing means a vacuum in it. So there is an error already in it.

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,

Ok Volume water is more or less volume of the flask. With density of air you get mass of air.
(Flask,stopper + air)  - mass of air is mass of flask, stopper

and then the mass of CO2 in the flask from that.

Get mass of CO2.

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

But this is molar Mass not mass of CO2 and the unit has to be g/mol

 

 

[Suspicious_boat_5926]

You're right. sorry. I meant 43.44 gmol is the molar mass of the CO2.

I was tired.

Okay, so

Nevermind, I'll just take a picture and upload it.

https://imgur.com/a/TPLbKOg

And yes, I argued with the prof about the discrepancy, but he said "its just a matter of the mass of water being so large that the error by the air is going to be 0.01g" or something.

Cant remember it well.

Anyway, it made no sense at all to me. But yes, we were supposed to

[This is mass(Flask,stopper  + water) - (Flask,stopper + air) = mass of water - ~~mass of air~~??]

Correct, Using 396.64 minus 131.4413 gets me mass of the water. Mass of the water / density gets me the .26573L.

Volume * density of the dry air, gets me the dry air that was put in at the beginning of the experiment, as 0.317g.

Using 0.317g and subtracting from 131.4413 gets me the mass of an empty (vacuum) flask with the stopper and plug, which therefore is subtracted from the CO2 flask of 131.6014 to get me mass of CO2 at 0.47712g.

At this point an equation m/V = PM / RT is used. I can plug in m, V, P, R, and so the remaining equation is 1.795507 = 0.041327 * M.

Then divide both sides by 0.041327, and get M = 43.44 g/mol, where M is the molar mass.

[Borek]

Volume * density of the dry air, gets me the dry air that was put in at the beginning of the experiment, as 0.317g.

Using 0.317g and subtracting from 131.4413 gets me the mass of an empty (vacuum) flask with the stopper and plug

Not exactly - you are ignoring the buoyancy. Internal air cancels out (mass of the air inside is identical with the buoyancy). No need to subtract 0.317 g here.

0.317 g is the buoyancy error of the weighing of a CONTENT of the flask. It will produce a small relative error in the flask volume, but a large relative error in the CO₂ mass.

You have ignored an error in the flask mass, error that depends on the volume of the glass itself (not the flask volume, but volume of the material from which the flask is made). But in the end it doesn't matter either, as it the same in all cases, so it cancels out when you subtract them to find out masses of the gas/liquid that fills the flask.