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### Topic: Chemical equations  (Read 2050 times)

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#### auksas

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##### Chemical equations
« on: July 19, 2023, 05:31:17 PM »
4.5 g of aluminum is mixed with sulfur. How many grams of aluminum sulfide were obtained?

0,17mol
4,5g                        x mol, x g
2Al+3S Al2S3
2mol 3mol          1mol

M(Al)=27g/mol
n(Al)=4,5g/27g/mol=0,17mol

x=0,17mol*1mol / 2 mol= 0,085mol
n(Al2S3)=0,085mol
M(Al2S3)=150g/mol
m(Al2S3)=0,085mol*150g/mol=12,75g

The answer of this task is 24,9 g Al2S3. Where am I wrong? I can solve other tasks using the same method, but this time I've got the wrong answer.

#### Hunter2

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##### Re: Chemical equations
« Reply #1 on: July 19, 2023, 06:18:42 PM »

#### auksas

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##### Re: Chemical equations
« Reply #2 on: July 20, 2023, 02:50:50 AM »
But why? When such steps to right solution must be done?

#### Hunter2

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##### Re: Chemical equations
« Reply #3 on: July 20, 2023, 02:52:19 AM »
I would say is a mistyping or mis calculation.

#### Borek

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##### Re: Chemical equations
« Reply #4 on: July 20, 2023, 03:02:01 AM »
Must be a mistake in the answer key.

Note though: don't round down intermediate results, only the final answer. Correct result is not 12.75g but 12.52g.

Technically it should be rounded down to 2 significant digits, so 13g in both cases. Some teachers treat significant digits almost religiously. In reality they don't matter much, kind of a poor man's way of expressing the accuracy. There are better and much more strict ways.
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#### auksas

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##### Re: Chemical equations
« Reply #5 on: July 20, 2023, 03:06:32 AM »
I would say is a mistyping or mis calculation.
Obviously. But I can't find where

#### auksas

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##### Re: Chemical equations
« Reply #6 on: July 20, 2023, 03:28:48 AM »
Must be a mistake in the answer key.

Note though: don't round down intermediate results, only the final answer. Correct result is not 12.75g but 12.52g.

Technically it should be rounded down to 2 significant digits, so 13g in both cases. Some teachers treat significant digits almost religiously. In reality they don't matter much, kind of a poor man's way of expressing the accuracy. There are better and much more strict ways.
Thanks