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Old Sep 22nd 2017, 07:01 PM   #1
xxxwinnie
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Exclamation Green Chemistry (Gas Stoichiometry)

For my chemistry lab, I reacted Mg powder with HCl to create H2 gas. I measured the volume of H2 gas created by salt water displacement by reading off an inverted test tube. I measured 14.0 mL and calculated the percent yield (used PV=nRT to find theoretical volume and divided it by the actual- 0.014L volume) which was 62.2%.

The next question that makes no sense is: "If you did not get 100% yield, could this be explained by the temperature of the gas? If so, given the yield you observed do you expect the gas temperature might have been above or below the temperature recorded for the salt bath water (the thermometer states 290 K)." Hint was to use ideal gas law and see volume of H2 gas in relation to gas temperature.

Based on my interpretation, the percent yield has nothing to do with the temperature of the gas produced. And by calculating the number of moles of H2 gas produced (using PV=nRT I got 5.9*10^-4 mol) and plugging into the PV=nRT equation, everything cancels out and I get 290K, which makes the gas temperature the same as the salt water temperature. I don't think this reaction involves equilibrium either.

Generally confused on how I should answer this. Should I say that the temperature of the gas is higher than the salt bath, and since temperature is directly proportional to volume, higher temperature (when compared to 'initial' temperature of salt water bath) would yield more volume of H2 gas, which was what we observed.

Please help me out in any way!
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Old Sep 22nd 2017, 08:08 PM   #2
oz93666
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I don't understand why a salt water bath is used to collect the hydrogen ??

Or why there should not be 100% yield ???

The gas collected will be warmer than the bath , so this will make you think you have a higher yield ...
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Old Sep 23rd 2017, 04:50 AM   #3
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This is collected over water so you need to subtract vapor pressure of watr from total pressure calculated over there...this will further bring down the percent yield.
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Old Sep 23rd 2017, 12:13 PM   #4
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We were assuming the water vapour as 1 atm, and there wasn't really any talk about subtracting vapor pressure. Either way, I feel like my temperature would be lower given that this is an exothermic reaction. How would I carry out the vapour pressure calculation you mentioned?
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Old Sep 23rd 2017, 07:36 PM   #5
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Originally Posted by xxxwinnie View Post
Either way, I feel like my temperature would be lower given that this is an exothermic reaction.
That doesn't make sense .... the reaction gives off heat ... the gas will be hotter !!

chemtopper is right , not all of the 14ml is hydrogen about 2.5% is water vapour ... find vapour pressure of water at the temperature of the gas , divided by atmospheric pressure , this is the fraction of water vapour.
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Old Sep 24th 2017, 03:15 AM   #6
chemtopper
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Originally Posted by xxxwinnie View Post
We were assuming the water vapour as 1 atm, and there wasn't really any talk about subtracting vapor pressure. Either way, I feel like my temperature would be lower given that this is an exothermic reaction. How would I carry out the vapour pressure calculation you mentioned?
PP(total) = P(water) + P(H2 gas)
Vapor pressure of water is given in standard tables at different temperatures.
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Old Sep 24th 2017, 08:58 AM   #7
xxxwinnie
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Originally Posted by oz93666 View Post
not all of the 14ml is hydrogen about 2.5% is water vapour ... find vapour pressure of water at the temperature of the gas , divided by atmospheric pressure , this is the fraction of water vapour.
Ah yes, the gas being hotter does make sense. How do I know that 2.5% is water? And do I use PV=nRT to find the vapour pressure of water? How would I find the number of moles, n? And divided by atmospheric pressure, which we were assuming as 1 atm, I will just use that?
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Old Sep 24th 2017, 09:05 AM   #8
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Originally Posted by chemtopper View Post
PP(total) = P(water) + P(H2 gas)
Vapor pressure of water is given in standard tables at different temperatures.
Although the salt water bath's thermometer read 290K, for this purpose I can still assume the temperature is 290K correct? I haven't actually learned how to do partial pressures yet (I am not sure why my school is giving me a lab on this before we even covered it), so to find the partial pressure of H2, I go total gas pressure (using PV=nRT, where P is unknown, V is 14 ml, n is 'moles of hydrogen (or better described, whatever my yield is)', R is constant, and T is 290K) minus standard pressure of (salt?) water at 290K divided by the total (do I use a fraction, or does adding linearly work?) to get the partial pressure of H2 gas?

Sorry for the long sentence, thanks so much for your help thus far!
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Old Sep 24th 2017, 09:14 AM   #9
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In addition, another question asked about which other factors can be used to determine how "green" this reaction is. Can I say rate of reaction (which determines the efficiency or yield) and the E-factor- mass of waste per mass of product? Does this make sense?
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