March 26, 2017

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Long but quick chemistry lab that I need confirmation on answers if possible?

OK so I have this lab that went pretty quick and smooth and I have an online assessment that I needed to put all my figures in. I am all complete but am having trouble with 3 problems that the assessment is saying is wrong and I am looking for someone to verify or explain what I am doing incorrect.

Here were the steps of the lab and my findings:

Determining the molecular weight of Acetone:

Step 1: Obtain a Dumas bulb from the Equipment menu.

Step 2: Record the volume of the bulb in liters (volume is marked on bulb). .056L

Step 3: Record the mass the empty bulb using an electronic balance. 50.g

Step 4: Place 20 ml of acetone into the bulb.

Step 5: Obtain a 600-ml beaker.

Step 6: Select Beaker and place about 500 ml of water into beaker or enough to cover neck of bulb.

Step 7: Combine bulb with beaker (using combine menu option).

Step 8: Obtain a hot plate from Equipment menu.

Step 9: Place combined Dumas bulb and beaker assembly onto hot plate.

Step 10: Turn on hot plate to maximum heating setting.

Step 11: Allow all acetone to boil away and wait 1-2 minutes before removing bulb from beaker.

Step 12: After removing bulb from beaker, allow Dumas bulb to cool until acetone vapors have condensed (2-3 minutes).

Step 13: Weigh bulb and condensed vapor using an electronic scale and record result. 50.1062g

Then the lab told me to use this info to answer the assessment:

(i) Volume of Dumas bulb (L): 56ml or .056L

(ii) Mass of empty Dumas bulb (g): 50g

(iii) Mass of Dumas bulb + condensed acetone (g):50.106

(iv) Mass of acetone gas required to fill bulb ((iii) - (ii)): .106

(v) Molecular weight of Acetone: Mass / n = Mass / (PV/RT) = (Mass *R *T) /(P * V) =.00183

Pressure: 1atm
Temperature: 373 Kelvin (boiling point of water)

Then Here was the question I needed to answer (which I am getting number 2,5 and 6 incorrect):

1)Weight of empty Dumas flask=50g
2)Weight of acetone=? I tried both 39.285 & .1062 and it says these are wrong
3)Weight of liquid vapor after condensing=.106
4)Moles of acetone vapor based on ideal gas law=.00183
5)Calculated molecular weight of acetone based on experiment. (Give in grams / mole).= I tried .072 and that's incorrect
6)%error from the theoretical molecular of acetone and the experimental value.=which to me seems like there is a 0% error

  • chemistry - ,

    I don't have a clue as to what 2 means.

    On 5, I would divide the mass (.106) by the number of moles (.--183). That gives something that is not your answer.

  • chemistry - ,

    0.106g / 0.00183 mol = 57.9 g/mol

    [Acetone = CH3-CO-CH3, mol. mass about 58]

    Not bad!

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