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Topic: Did I completely bomb this lab or am doing the calculations wrong?  (Read 8886 times)

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Offline toadesque

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Did I completely bomb this lab or am doing the calculations wrong?
« on: November 14, 2008, 04:23:55 AM »
Ok obviously I know the GFW of CO2 is 44.01

But I did an experiment and the write up is asking for the GFW of CO2 from my data.

Well...here's my data

Mass of CO2 (g): .1532
Volume of CO2 (L): .27156
Gas pressure: .946
Temperature: 22.5 (+273 for k)

I am looking at my lab write up and I'm not sure but it looks like this is the formula to get the GFW

moles = g / GFW (i dont know why this was there but whatever)

GFW = gRT / PV


Ok I did all the calculations but my answer is WEIRD.

I got 1.0705

The actual GFW is 44.01. So what the hell is going on? Does this mean I completely bombed the experiment or am I calculating wrong?
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Offline Borek

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #1 on: November 14, 2008, 05:00:55 AM »
What does GFW stand for?

Have you use correct R value? With the correct units?
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Offline AWK

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #2 on: November 14, 2008, 05:06:59 AM »
units of pressure are missing.
If atm - check your data
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Offline toadesque

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #3 on: November 14, 2008, 05:23:42 AM »
Yeah sorry I messed up some of the data

GFW is gram formula weight (molar mass)

here are the right numbers



Mass of CO2 (grams): .1532
Volume of CO2 (liters): .27699
Gas pressure is .942 atm
Temperature: 23.5 (+273 for k)


What would the R be? 8.314? I tried plugging in and my answer is non sense. Some other guy I talked to did it and his answer was a lot closer to 44.01 than mine was. I don't know what I'm doing wrong.

In the mean time if you guys don't feel like dealing with this one. Could you check my work on this problem?

What volume of CO2 could be generated from 3.0 g of marble chips at ordinary lab conditions (20 degrees celsius and 710 torr)

I did

(3/100) (8.314) (293) / 94658

and got .77 L

Is that right?
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Offline enahs

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #4 on: November 14, 2008, 08:35:14 AM »
How did you measure the mass of a gas?
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Offline AWK

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #5 on: November 14, 2008, 09:20:26 AM »
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Offline Borek

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #6 on: November 14, 2008, 11:49:56 AM »
Quote from: toadesque on November 14, 2008, 05:23:42 AM
Mass of CO2 (grams): .1532
Volume of CO2 (liters): .27699
Gas pressure is .942 atm
Temperature: 23.5 (+273 for k)

It doesn't give correct molar mass. Not 1.07 as you have posted earlier, but still about 3 times too low.

Quote from: toadesque on November 14, 2008, 05:23:42 AM
and got .77 L

Looks OK to me.
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Offline toadesque

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #7 on: November 14, 2008, 01:07:49 PM »
There's just no way my answer could be that off. I did the lab perfectly and I got the correct results. Maybe it's too confusing to explain online...can't blame you guys for that. I guess I'll have to ask my professor...too bad my lab is gonna be marked as late though...
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Offline sjb

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #8 on: November 14, 2008, 01:44:01 PM »
Quote from: toadesque on November 14, 2008, 05:23:42 AM
Yeah sorry I messed up some of the data

GFW is gram formula weight (molar mass)

here are the right numbers



Mass of CO2 (grams): .1532
Volume of CO2 (liters): .27699
Gas pressure is .942 atm
Temperature: 23.5 (+273 for k)


What would the R be? 8.314? I tried plugging in and my answer is non sense. Some other guy I talked to did it and his answer was a lot closer to 44.01 than mine was. I don't know what I'm doing wrong.

In the mean time if you guys don't feel like dealing with this one. Could you check my work on this problem?

What volume of CO2 could be generated from 3.0 g of marble chips at ordinary lab conditions (20 degrees celsius and 710 torr)

I did

(3/100) (8.314) (293) / 94658

and got .77 L

Is that right?

Let's try a different tack.

Why have you used 8.314 for your value of R in the first example, when your pressure was quoted in atm, and also in the second example, with pressure in torr?

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Offline toadesque

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #9 on: November 14, 2008, 01:54:23 PM »
lol....huh? which part are you referring to. the marble chip one is an entirely different question.

I don't understand the mass one at all though.
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Offline Borek

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #10 on: November 14, 2008, 02:41:13 PM »
Quote from: toadesque on November 14, 2008, 01:07:49 PM
There's just no way my answer could be that off.

Unfortunately, it is.

Where did you get CO2 mass from?
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Offline sjb

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #11 on: November 14, 2008, 04:04:03 PM »
Quote from: toadesque on November 14, 2008, 01:54:23 PM
lol....huh? which part are you referring to. the marble chip one is an entirely different question.

I don't understand the mass one at all though.

R has a different numerical value depending on the units used to measure it.

It's like saying I am either 6 (ft) or 1.8 (m) tall.
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Offline toadesque

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #12 on: November 15, 2008, 05:57:04 PM »
Okay I talked to bunch of kids from class and they were having the same problems. All of us got non sense answers that weren't even close to 44.01 GFW.

As for R, I remember looking at the lab handout and it said R is .0821. I think. haha.

My percent error is going be too high. How is it possible to be that off? How could all of us have done the lab that completely wrong?
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Offline Borek

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #13 on: November 15, 2008, 06:41:54 PM »
Hard to even guess anything not knowing the procedure.
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Offline toadesque

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Re: Did I completely bomb this lab or am doing the calculations wrong?
« Reply #14 on: November 17, 2008, 12:04:51 AM »
Quote
Where did you get CO2 mass from?

Subtracted the weight of the "empty" flask with air in it from the weight of just the flask. That gives the mass of CO2
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