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发表于 2012-6-11 02:29:21 | 查看: 793| 回复: 0
题目:
When a 5.00 g sample of ethanol (C2H5OH) undergoes complete combustion in a sealed 10.0 L chamber with excess oxygen gas at 298K, the total pressure after the reaction is 4.51 atm. Calculate the mass of oxygen gas (in grams) present in the chamber before the reaction.


(The other test version had 7.00 grams to start with).

Here’s how to approach it:
1.       Write a balanced equation for the combustion of ethanol.

2.       Calculate the number of moles of gas formed by complete combustion.

3.       Use the gas law to calculate the number of moles of gas present at the end – this is “unreacted oxygen (x) + gases formed by combustion”

4.       Solve for x – this is the moles of unreacted oxygen

5.       Find the number of moles of oxygen consumed by the reaction

6.       Add moles of unreacted oxygen to moles of oxygen consumed = total moles of oxygen

7.       Convert moles to grams

8.       Answer = 52.1 grams of oxygen before the reaction
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