September 27, 2023, 06:58:41 PM
Forum Rules: Read This Before Posting

Topic: Chemical equations  (Read 919 times)

0 Members and 1 Guest are viewing this topic.

auksas

• New Member
• Posts: 7
• Mole Snacks: +0/-1
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

• Sr. Member
• Posts: 1994
• Mole Snacks: +153/-45
• Gender:
• Vena Lausa moris pax drux bis totis
Re: Chemical equations
« Reply #1 on: July 19, 2023, 06:18:42 PM »

auksas

• New Member
• Posts: 7
• Mole Snacks: +0/-1
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

• Sr. Member
• Posts: 1994
• Mole Snacks: +153/-45
• Gender:
• Vena Lausa moris pax drux bis totis
Re: Chemical equations
« Reply #3 on: July 20, 2023, 02:52:19 AM »
I would say is a mistyping or mis calculation.

Borek

• Mr. pH
• Deity Member
• Posts: 27420
• Mole Snacks: +1781/-408
• Gender:
• I am known to be occasionally wrong.
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.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

auksas

• New Member
• Posts: 7
• Mole Snacks: +0/-1
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

• New Member
• Posts: 7
• Mole Snacks: +0/-1
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

Barbara Lewis

• New Member
• Posts: 3
• Mole Snacks: +0/-0
Re: Chemical equations
« Reply #7 on: August 14, 2023, 04:24:55 AM »
Yes, I also think that you gave the right variant and the give one was a typo.
Given:
4.5 g Al
Balanced equation:
2Al + 3S → Al2S3
Moles of Al:
4.5 g Al / 27 g/mol Al = 0.17 mol Al
Using stoichiometry:
0.17 mol Al * (1 mol Al2S3 / 2 mol Al) = 0.085 mol Al2S3
Grams of Al2S3:
0.085 mol Al2S3 * 150 g/mol Al2S3 = 12.75 g Al2S3
Therefore, unless there is additional information missing, the final mass of Al2S3 obtained from reacting 4.5 g Al with excess S should be 12.75 g.