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Hi so this a very good question. This question is about the calculations of stoichiometric ratios. I will be assuming you know how chemical reactions occur and what happens in them (Understanding this concept is very important before you start learning to do calculations as it will make the chemistry calculations very intuitive for you).
We know that the stoichiometric ratio of Iron is 4 and for the product Fe2O3 is 2. This essentially means for every 2 lots (or moles) of Fe2O3 produced 4 lots (or moles) of Iron is used. In the previous step number of moles of Fe2O3 is calculated, so to calculate the number of moles of Fe used up in the reaction it will be 4/2 x number of moles of Fe2O3 produced.
(number of moles of Fe2O3/ moles of Fe = 2/4) Rearrange this to get the above equation.
We know that the stoichiometric ratio of Iron is 4 and for the product Fe2O3 is 2. This essentially means for every 2 lots (or moles) of Fe2O3 produced 4 lots (or moles) of Iron is used. In the previous step number of moles of Fe2O3 is calculated, so to calculate the number of moles of Fe used up in the reaction it will be 4/2 x number of moles of Fe2O3 produced.
(number of moles of Fe2O3/ moles of Fe = 2/4) Rearrange this to get the above equation.
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ahh i get what your saying, thanks so much for the help
Balance the equation using the ratios and then using the ratios you will be able to determine. Again understanding how chemical reactions occur will help you a lot I recommend you go back and relearn.
True will do
I tried balanced them i got 2C2H6 +7O2 -> 6H2O+4CO2
its in the ratio of 7:2 for oxygen and so i put 2/7 time 16 ( the molar mass of oxygen) but i get the wrong answer
When chemical reactions happen the reactants react in number of moles and not in mass. So, Molar mass is not required in this question. What that balanced reaction equation tells us is that you need 2 moles of ethane to react with 7 moles of oxygen gas to produce 6 moles of H20 and 4 moles of CO2. This means 2 moles of ethane reacts with 7 moles of oxygen. We have to figure out how many moles of oxygen is needed to react with 1 mole of ethane. Hence, you just divide 7 by 2 that means 3.5 moles are needed. Please confirm if that is the answer.
The proper working out:
7/2 = moles of oxygen/moles of ethane (Stoch Ratio)
We know moles of ethane is 1 so,
7/2 = moles of oxygen/1
Therefore, moles of oxygen = 7/2 = 3.5 moles
I get it now, the answer is 2.5 I might have balanced the equation incorrectly
Your balancing seems fine I think this textbook might have the wrong answer
If you have a look at this older thread, you will notice that the user got the same answer as defeatHSC above, meaning that it is possible that your textbook may contain an incorrect answer:
boredofstudies.org
Conquering Chem help: Moles
there's a few questions i need help with related with moles 30. a] Ethane (C2H6) burns in oxygen to form carbon dioxide and water. how many moles of oxygen are needed to react with one mole of ethane? How many moles of: (i) CO2 (ii) water are produced? answer: a) 3.5; (i) 2 and (ii) 3 b]...
boredofstudies.org
They most likely have written the equation for ethyne (C2H2) which has been mistaken for ethane (C2H6)
Ethene combustion would be:
C2H2(g) + 5/2 O2(g) --> 2CO2(g) + H2O(l)
The answer for ethyne would be 2.5 moles of O2
For ethane, the answer would be 3.5 moles