Similar questions in berkeley, different answers?

[nolookpass]

Chapter 1 in Gen Chem (Stoichiometry) question 26 vs. Chapter 7 in Gen Chem (Phase and Phase Changes) question 73.


Chapter 1 question 26 asks
26. If in the second experiment the organic vapor had not
fully displaced all of the air from the flask by the time
the heat was removed from the flask, how would the
results have been affected?

The answer was the mass of the unknown liquid collected would be too small and the molecular mass would be too low.

Chapter 7 question 73 asks
What would be the result if the heating source were
removed before all of the organic liquid had completely
vaporized?

The answer was the mass would be too high and the molecular mass would be too high

Not sure how these two questions are different

Thanks

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[kraskadva]

I have an idea, but the passages for each would be helpful...because context is important, ya' know.

 

[nolookpass]

Do you have the Berkeley books? Not sure how to post the entire passage.

 

[kraskadva]

No, don't have the books.
Does your phone take pictures?

 

[nolookpass]

No, don't have the books.
Does your phone take pictures?

Portion of passage for question 26 that relates to the question

In a second experiment, the researcher places a 5.0-mL
aliquot of the unknown liquid into a capped 1.00-liter flask.
The cap has a tiny hole in the top, and the empty flask with
cap weighs exactly 120.00grams. The compound is heated
until it reaches a gentle boil. The vapor escapes through the
tiny pore in the cap. The liquid continues boiling at 31"C,
until none of it remains visible in the flask. The heat source
is removed from the flask, and the contents are allowed to
cool back to ambient temperature. As the flask cools, the
vapor in it condenses into a small pool of liquid at the base
of the flask.
The flask and cap are then massed with the condensed
liquid present. The entire system is found to have a mass of
exactly 122.32 grams. This means that the mass of the
liquid is 2.32 grams. It is assumed that at the moment when
the heat source was removed, the flask was completely filled
with vapor from the liquid and that all of the air originally in
the flask was displaced. Table 1 lists


Passage for question 73

The Dumas experiment involves filling a flask of known
mass and volume with an organic liquid. The organic liquid
is heated, so that it begins to vaporize. Assuming that the
air in the flask is lighter than these organic vapors, the air is
displaced out of the flask by the organic vapors through a
small hole at the top of the flask. The ideal flask is a
sphericalflask with a stopcock attached, but a good substitute
would be any flask with a foil cap that has a small pinhole in
the foil cap. Foil is chosen in lieu of plastic or rubber,
because organic solvents will dissolve both plastic and
rubber. The solution is heated by flame until the last trace of
liquid residue has evaporated.
Once the liquid has completely evaporated, it is assumed
that the flask is filled entirely with organic vapors. As the
flask cools, the vapors condense back into a liquid, and air
flows back into the flask through the pinhole. Once the flask
is back to its initial temperature (ambient temperature), the
combined mass of the flask and condensed organic liquid is
measured. The increase in mass over the initial weighing of
the empty flask can be attributed to the organic liquid in the
flask. The small amount of air displaced by the organic
liquid is assumed to be negligible

 

[kraskadva]

Ok, reverse order...

Chapter 7 question 73 asks
What would be the result if the heating source were
removed before all of the organic liquid had completely
vaporized?

The answer was the mass would be too high and the molecular mass would be too high

Passage for question 73

The Dumas experiment involves filling a flask of known
mass and volume with an organic liquid. The organic liquid
is heated, so that it begins to vaporize. Assuming that the
air in the flask is lighter than these organic vapors, the air is
displaced out of the flask by the organic vapors through a
small hole at the top of the flask. The ideal flask is a
sphericalflask with a stopcock attached, but a good substitute
would be any flask with a foil cap that has a small pinhole in
the foil cap. Foil is chosen in lieu of plastic or rubber,
because organic solvents will dissolve both plastic and
rubber. The solution is heated by flame until the last trace of
liquid residue has evaporated.
Once the liquid has completely evaporated, it is assumed
that the flask is filled entirely with organic vapors. As the
flask cools, the vapors condense back into a liquid, and air
flows back into the flask through the pinhole. Once the flask
is back to its initial temperature (ambient temperature), the
combined mass of the flask and condensed organic liquid is
measured. The increase in mass over the initial weighing of
the empty flask can be attributed to the organic liquid in the
flask. The small amount of air displaced by the organic
liquid is assumed to be negligible

The Dumas method is pretty straightforward- using the ideal gas law to determine the molecular mass through a known volume and measured mass.
If the liquid had not been completely vaporized, then the final measurement would have been too high because...
What you want is the mass of the vapor that would fill the flask.
What you would get is the mass of the vapor + mass of unvaporized liquid.

Now, the reason I did these in reverse order...these 2 are the same set up and the same thing being measured. This question though is trying to be tricky. The one above seems pretty intuitive to most people, because you could see that happening. This one though is asking you to imagine things you can't see. The question says nothing about the liquid not being entirely vaporized, only that it doesn't displace all the air. Which means it's not entirely filling the volume of the flask (because it's a big flask and they only started with 5mL) and some of that space is being occupied by air. Air will weigh less than the organic solvent, so mass (less solvent +air) < mass (more solvent alone).

Chapter 1 question 26 asks
26. If in the second experiment the organic vapor had not
fully displaced all of the air from the flask by the time
the heat was removed from the flask, how would the
results have been affected?

The answer was the mass of the unknown liquid collected would be too small and the molecular mass would be too low.

In a second experiment, the researcher places a 5.0-mL
aliquot of the unknown liquid into a capped 1.00-liter flask.
The cap has a tiny hole in the top, and the empty flask with
cap weighs exactly 120.00grams. The compound is heated
until it reaches a gentle boil. The vapor escapes through the
tiny pore in the cap. The liquid continues boiling at 31"C,
until none of it remains visible in the flask. The heat source
is removed from the flask, and the contents are allowed to
cool back to ambient temperature. As the flask cools, the
vapor in it condenses into a small pool of liquid at the base
of the flask.
The flask and cap are then massed with the condensed
liquid present. The entire system is found to have a mass of
exactly 122.32 grams. This means that the mass of the
liquid is 2.32 grams. It is assumed that at the moment when
the heat source was removed, the flask was completely filled
with vapor from the liquid and that all of the air originally in
the flask was displaced. Table 1 lists

Does that help?

 

[nolookpass]

Thanks appreciate it!