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### Topic: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY  (Read 12746 times)

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#### Jasmine

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##### HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« on: July 08, 2008, 07:05:08 PM »

Hi,

I was wondering if I could post a rather difficult Stoichiometry problem. It involves the combustion of three different hydrated salts: Na2CO3.10H2O, MgSO4.7H2O and CuSO4.5H2O:

You are given a mass of three hydrated salts: (see above). The total mass of the mixture is 12.123 grams. When the mixture is gently heated, the following two reactions occur:

Na2CO3.10H2O (s) + _______________> Na2CO3.7H2O (s) +3H2O (g)

MgSO4.7H2O (s) ______________> MgSO4.H2O (s) +6H2O (g)

After these reactions are complete, the mass of the mixture has decreased to 9.049 grams. This mixture is then heated more strongly and the following additional reactions occur:

Na2CO3.7H2O (s) ___________________> Na2CO3 (s) + 7H2O

MgSO4.H2O (s) _____________________> MgSO4 (s) + H2O (g)

CuSO4.5H2O (s) _____________________> CuSO4 (s) + 5H2O (g)

After this final heating, the mass of the mixture has decreased to 6.412 grams. From this information, calculate the masses of each of the three compounds in the original mixture.

ANSWERS GIVEN: 1.374 grams of Na2CO3,.10H2O, 6.418 grams of MgSO4.7H2O and 4.331 grams of CuSO4.5H2O.  I did calculate the Molecular Weights of each one of these compounds at every different stage of the reactions.  Then I tried to calculate how much water was lost at each stage, but my answers are not the same.

Any ideas out there?  I know a lot of Chemists and Chemical engineers peruse this site on a regular basis.  I figured this would be the place to go, rather than Yahoo Answers, for a question this difficult.

Thanks,

Jasmine
Jazz

#### vhpk

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##### Re: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« Reply #1 on: July 09, 2008, 12:12:41 AM »
To solve this problem, you must solve the three - variable simultaneous equations.
I want to notice you that, the heating itself dehydrate the salts and no chemical substance react to take out water.
With the above data, you can form 3 equations:
- The relationship about the mass
- In experiment I, the decrease in mass that is water evolved
- In experiment II, the similar to experiment I.
Hence, solve them to get the answer.
Good luck
Genius is a long patience

#### Borek

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##### Re: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« Reply #2 on: July 09, 2008, 02:01:00 PM »
It involves the combustion

Roasting, not combustion. Follow vphk hints and you should get there. It is about writing system of equations in three unknowns and solving it.
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#### sameeralord

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##### Re: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« Reply #3 on: July 10, 2008, 04:46:25 AM »
I like to know the answer to this question as well. I tried I couldn't do it. When you guys are talking about variables. Which three varibales are you talking about. The compounds of the orginal mixtures or waters. I couldn't get three equations either. I used water as a variable and got 2 equations. I don't think it is right.

X- Water of Na2CO3.10H2O
Y- Water of MgSO4.7H2O
Z- Water of CuSO4.5H2O

7X+Y+5Z = 2.637
10X+7Y+5Z= 5.771

This doesn't seem right.Can our helpers elaborate on their help please. I'm too interested in this question!!

#### Borek

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##### Re: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« Reply #4 on: July 10, 2008, 04:59:14 AM »
Your variables are initial masses of all three compounds.
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#### sameeralord

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##### Re: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« Reply #5 on: July 10, 2008, 05:03:52 AM »
Your variables are initial masses of all three compounds.

ok. Thank you for the quick response . But then I'm really confused with the waters attached.

X-  Na2CO3.10H2O
Y- MgSO4.7H2O
Z- CuSO4.5H2O

X+Y+Z= 12.123

Where do I go from here.  I can't go 7X and stuff because only the waters are multiplied in this not the whole thing.  Help would be great. Thanks!!

#### Borek

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##### Re: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« Reply #6 on: July 10, 2008, 05:14:00 AM »
If there was initiall x g of Na2CO3.10H2O, how much Na2CO3.7H2O was left after dehydration? Hint: has the number of moles changed?
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#### sameeralord

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##### Re: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« Reply #7 on: July 10, 2008, 05:20:37 AM »
If there was initiall x g of Na2CO3.10H2O, how much Na2CO3.7H2O was left after dehydration? Hint: has the number of moles changed?

I'm off trying to work out this question. I'll think about your hint and try to work it out. Thanks a lot for the help . I'll be back with my findings.

#### sameeralord

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##### Re: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« Reply #8 on: July 10, 2008, 07:13:21 AM »
Well finally worked it out. Thanks for the hints Borek you were right.   . Thanks again!! . Well as far as the simulataneous equations go I'm amazed this is a chemistry question. This is more like a maths question I'm amazed whole numbers weren't involved. Here is how you do it.

X-  Na2CO3.10H2O
Y- MgSO4.7H2O
Z- CuSO4.5H2O

X+Y+Z= 12.123

Well to find the numbers for other equations you have to use stoichimetry.

Example how I got the 2nd equation

Na2CO3.10H2O (s) + _______________> Na2CO3.7H2O (s) +3H2O (g)

As Borek pointed out relationship between the moles of Na2CO3.10H2O and Na2CO3.7H20 is 1:1. Hence n=m/mr  which is X/286. That multiplied by the molar mass of Na2CO3.7H2O  you get the mass of that compound which is 232/286 x. Using this method you can work out the three equations. You should get

X+Y+Z= 12.123

(232/286)X + (138.4/246.4)Y + Z = 9.049

(106/286)X + (120.4/246.4)Y + (159.7/249.7)Z = 6.412

Now you have to solve these equations to get the answers.

http://math.cowpi.com/systemsolver/3x3.html I used the online simultaneous equation solver because I was bit lazy to work out these numbers. Make sure you enter these numbers in decimal form in that website.

Hope this helped . My first real contribution in the forum

#### Borek

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##### Re: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« Reply #9 on: July 10, 2008, 09:14:07 AM »
Well finally worked it out.

Very good

Quote
Thanks for the hints Borek you were right.

Happens now and then Sometimes I am wrong.

Just in case - please read forum rules. In the future please try to not list final versions of equations, as you are doing someone else homework this way.
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#### sameeralord

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##### Re: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« Reply #10 on: July 10, 2008, 09:25:40 AM »
Well finally worked it out.

Very good

Quote
Thanks for the hints Borek you were right.

Happens now and then Sometimes I am wrong.

Just in case - please read forum rules. In the future please try to not list final versions of equations, as you are doing someone else homework this way.

Will do . Getting the question wrong and retrying is the best way to learn as I just found out .

#### Jasmine

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##### Re: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« Reply #11 on: July 15, 2008, 07:07:23 AM »
Hi again,

I want to thank Borek and SameeraLord for their help in solving this question for me.

Also along the lines of Stoichiometry, is there such a thing as, say, a book collection of many different types of Stoichiometry problems ranging from easy to the most difficult that I could purchase?  Sort of like a collection of card games or crossword puzzles.  Most stoichiometry problems I can handle but that one had me stumped.

Thanks,
Jaz

Jazz

#### Borek

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##### Re: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« Reply #12 on: July 15, 2008, 07:39:31 AM »
Not much, but several of very different types.

http://www.chembuddy.com/?left=balancing-stoichiometry-questions&right=toc

Could be your question will make its way there
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#### macman104

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##### Re: HYDRATED SALTS QUESTION CONCERNING STOICHIOMETRY
« Reply #13 on: July 15, 2008, 09:48:44 AM »
Google "stoichiometry worksheets", there are tons of hits for posted problems and entire worksheets.  Also, change the keyword to "handouts" instead of worksheets for more hits.