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Author: Subject: Stoichiometry question: Al₂S₃+6H₂O→2Al(OH)₃+3H₂S
hechz
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Stoichiometry question: Al₂S₃+6H₂O→2Al(OH)₃+3H₂S

Al₂S₃+6H₂O→2Al(OH)₃+3H₂S

Concerning the above reaction I am asked what mass of Al(OH)₃ will be produced when 14.2g of Al₂S₃is fully reacted.

My first step was to convert the mass to the count of moles.

14.2g
----------------- = 0.115 mol of Al₂S₃
123.177g/mol

Then I determine the number of moles of the product, using the coefficients of the balanced equation:

⎛ 2 mol/Al(OH)₃ ⎞
0.115 mol/Al₂S₃⎜ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯  ⎟ = 0.23 mol/Al(OH)₃
⎝ 1 mol/AlS₃    ⎠

0.23(78.006)=17.94g/Al(OH)₃

I am failing to see wherein my error lies.

[Edited on 8-10-2009 by hechz]

[Edited on 8-10-2009 by hechz]
psychokinetic
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Ugh, I tried for you but I got 12.96g

Sorry, I'll keep trying.

“If Edison had a needle to find in a haystack, he would proceed at once with the diligence of the bee to examine straw after straw until he found the object of his search.
I was a sorry witness of such doings, knowing that a little theory and calculation would have saved him ninety per cent of his labor.”
-Tesla
woelen

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14.2 grams of Al2S3 is not 0.115 mol, but 0.0946 mol. With this info you should be able to find your mistake.

The art of wondering makes life worth living...
Want to wonder? Look at https://woelen.homescience.net
Alkyrisk
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Molar mass for Al2S3 is 150.158 g/mol. Why? because

26.982 x 2 = 53.964

32.065 x 3 = 96.195
150.159 - 123.177(your hypothetical molar mass) =
????...??? exactly 26.982 (What these numbers mean? This is the question!)
Nicodem
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I don't know what kind of acrobatic arithmetics you are doing above and I'm not going to search for where you made a mistake but I think Woelen already pointed it out. Instead of using numbers which leads to confusion, use variables instead. Like this way:

m[Al(OH)3] = M[Al(OH)3] * 2 * m[Al2S3] / M[Al2S3]

When m[Al2S3] = 14.2 g then m[Al(OH)3] = 14.75 g just like your textbook claims!
Alkyrisk
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I want to find the origin of mistake. He took only one aluminium molecule instead 2.
M Al2S3 = 150.158, M (AlS3) = 123.177. The confusion may be derived from my acrobatic poor english.

 Sciencemadness Discussion Board » Fundamentals » Beginnings » Stoichiometry question: Al₂S₃+6H₂O→2Al(OH)₃+3H₂S Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Responsible Practices   » Miscellaneous   » The Wiki Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Radiochemistry   » Computational Models and Techniques   » Prepublication Non-chemistry   » Forum Matters   » Legal and Societal Issues