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In a crucible, we added copper carbonate and the total weight of both is 37.04g. After, we heated the crucible with the copper carbonate. It was easy to judge when the reaction had taken place as the colour of the substance had changed from the light green of copper carbonate to the black of copper oxide.After we observed this, we weighed the copper oxide and we notified the result which was 36.42g. We should have heated and weighed the crucible again, but we weren't able because it was broken(due to temperature change). But theoretically, which would be approximately the value of the second weighing?I'm curious because after searching on the Internet, I have seen both increase and reduction of the remaining mass.

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  • $\begingroup$ Do you know the weight of the empty crucible? Also, I assume that you mean the total mass of the crucible plus the copper oxide was 36.42g? $\endgroup$ – airhuff Mar 16 '17 at 16:38
  • $\begingroup$ Unless I'm missing something, the mass should not have changed with a second heating, assuming all of the copper carbonate was oxidized during the first heating. Were the conditions of the second heating supposed to be the same as the first? Are you supposed to calculate the total amount of copper or anything else? $\endgroup$ – airhuff Mar 16 '17 at 16:53
  • $\begingroup$ @airhuff Yes, the total mass of the crucible plus the copper oxide was 36.42g. The weight of the empty crucible was 34.86g and the weight of the copper carbonate was 2.18g. $\endgroup$ – chemistrylove Mar 16 '17 at 16:57
  • $\begingroup$ The instruction said that for the second time we should heat it for 1 minute and then we should weigh it again. It also said that if the two measurements differed more than 0.03g we should heat it for 1 minute again. I'm supposed to think which should be the theoretical value of the second weight. $\endgroup$ – chemistrylove Mar 16 '17 at 17:00
  • $\begingroup$ Cool, I get it now. The repeated heatings were just to ensure that you had oxidized all of the copper carbonate to copper oxide. If you heated it a second time and the mass decreased, that would just mean that some of the copper carbonated had not been oxidized by the first heating. Then you would just keep heating until the mass quit decreasing, meaning that you had definitely oxidized all of it. So the answer to you question is simply that the mass theoretically should not have decreased at all with a second heating, unless some of the copper carbonate had not been oxidized the first time. $\endgroup$ – airhuff Mar 16 '17 at 17:14
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The repeated heatings were just to ensure that you had oxidized all of the copper carbonate to copper oxide. If you heated it a second time and the mass decreased, that would just mean that some of the copper carbonated had not been oxidized by the first heating. Then you would just keep heating until the mass quit decreasing, meaning that you had definitely oxidized all of it.

So the answer to you question is simply that the mass theoretically should not have decreased at all with a second heating, unless some of the copper carbonate had not been oxidized the first time.

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  • $\begingroup$ I'm sorry for asking again, but is it right to say that the theoritical amount of Cu we extract will be the same as the mass of Copper Carbonate we added? if the whole copper carbonate oxidized to copper oxide? $\endgroup$ – chemistrylove Mar 16 '17 at 19:18
  • $\begingroup$ There should be no loss of Cu, so the moles of CuO you made should be the same as the moles of CuCO3 that you started with. So, take the mass of CuCO3 you started with and divide that my it's molecular weight (123.6 g/mol) to get the moles of CuCO3 (which is the same as the moles of CuO and CuC3 in this experiment). Then multiply the moles of CuCO3 by the molecular weight of CuO, and this should give you the theoretical mass of CuO that you should have gotten if the experiment all worked correctly. Don't hesitate to ask if you have more questions. $\endgroup$ – airhuff Mar 16 '17 at 19:30

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