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Offline luiseduardo

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Stoichiometric calculation
« on: August 15, 2009, 12:00:31 PM »
The table below shows the result of three experiments, which are to make an electric spark through a mixture of hydrogen and oxygen gases (synthetic water)

| Quantity early in grams | grams final volume |
| | Oxygen | Water | Excess |
| 1 | 3.0 | 30.0 | X | 6.0 (oxygen) |
| 2 | 0.2 | 2.0 | 1.8 | Y (oxygen) |
| 3 | 5.0 | 20.0 | W | Z (hydrogen) |

Calculate the values of X, Y, W and Z that satisfy both the laws of Lavoisier and Proust

Offline Borek

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Re: Stoichiometric calculation
« Reply #1 on: August 15, 2009, 12:02:06 PM »
Please read forum rules.
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Offline luiseduardo

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Re: Stoichiometric calculation
« Reply #2 on: August 15, 2009, 12:04:27 PM »
Ok, but do you know how to help me ?

Offline Dan

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Re: Stoichiometric calculation
« Reply #3 on: August 15, 2009, 12:08:27 PM »
You must show some kind of attempt at the question.

Start with the balanced equation for the reaction.
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Offline luiseduardo

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Re: Stoichiometric calculation
« Reply #4 on: August 15, 2009, 12:16:00 PM »
Ok, I found x = 27 ; y = 0,4, but w and z I could not solve. Could anyone do this question to help me to understand ?

Offline Dan

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Re: Stoichiometric calculation
« Reply #5 on: August 15, 2009, 12:22:06 PM »
I agree with your values of X and Y. If you could calculate X and Y, I don't understand why you can't calculate W and Z - the method is exactly the same. Can you show how you did your calculations?

We will guide you to the answer, not do it for you, that way you will learn.
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Offline luiseduardo

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Re: Stoichiometric calculation
« Reply #6 on: August 15, 2009, 12:25:02 PM »
Thanks  :D

My method is 2/20 = 1,8/w   ---> w = 18

1,8/18 = 0,4/z  ---> z = 4

but 5 + 20 = 25 - 4 = 21#18 (DIFFERENT)

is it my difficult.
Can you help me ?

Offline Dan

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Re: Stoichiometric calculation
« Reply #7 on: August 15, 2009, 12:30:33 PM »
My method is 2/20 = 1,8/w   ---> w = 18

1,8/18 = 0,4/z  ---> z = 4

but 5 + 20 = 25 - 4 = 21#18 (DIFFERENT)

This makes no sense to me at all. Start from the beginning,

1. What is the balanced chemical equation for the reaction of hydrogen and oxygen?
My research: Google Scholar and Researchgate

Offline luiseduardo

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Re: Stoichiometric calculation
« Reply #8 on: August 15, 2009, 12:40:35 PM »
I don't know very well. It's my question. I don't understand why w = 18 and z = 4 in my anwer using proust's law. Do you know how to do or correct my answer ?

Offline Dan

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Re: Stoichiometric calculation
« Reply #9 on: August 15, 2009, 01:30:49 PM »
I don't understand why w = 18 and z = 4 in my anwer using proust's law.

It's because in the first two experiments oxygen is in excess and in experiment 3 hydrogen is in excess.

I know how to solve this problem and I am trying to help you.

Start with the chemical equation (balanced), what is it?
My research: Google Scholar and Researchgate

Offline luiseduardo

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Re: Stoichiometric calculation
« Reply #10 on: August 15, 2009, 01:57:44 PM »
ok, 5 + 20 = 25 and now ?
I don't know how to do. What's the correct answer ?

Offline Borek

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Re: Stoichiometric calculation
« Reply #11 on: August 15, 2009, 03:36:19 PM »
Dan have asked you for the balanced reaction equation. Three times. He knows what he is doing.
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Offline luiseduardo

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Re: Stoichiometric calculation
« Reply #12 on: August 15, 2009, 07:43:31 PM »
That's ok, Borek. But I don't know how to do balanced reaction equation. I thought my anwer was correct. But now I'm confused. What I need to do ?
Is my answer wrong ? And how to correct it ?

Offline Borek

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Re: Stoichiometric calculation
« Reply #13 on: August 16, 2009, 03:32:30 AM »
Hm, I think I know what is going on - you are starting your chemistry course and you have not yet learnt how to do stoichiometry, but they are trying to show you how such things could be done in the late 18th century.

That's assuming you were not sleepind during lectures ;)

In each case sum of masses of products and excess reactants must be identical to the initial mass of reactants, and ratio of masses must be the same. Try to write system of simultaneous equations that will describe these conditions, then solve them.
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Offline luiseduardo

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Re: Stoichiometric calculation
« Reply #14 on: August 16, 2009, 09:40:53 AM »
ok, but I don't know where's my mistake.

My calculate is: W = 25 - Z

2/20 = 1,8/25-Z
Z = 7

25 - 7 = 18

But 1,8/18 = 0,4/7      ------> 12,6 = 7,2 (Different)

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