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Topic: Mass fraction?  (Read 6359 times)

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Offline Johulus

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Mass fraction?
« on: January 28, 2015, 04:00:52 AM »
I don't have a clue how to start. Mass of mixture of calcium carbonate and calcium hydroxide has shrunk after flaming to 60% of initial mass of mixture. Calculate the mass fraction of calcium carbonate and calcium hydroxide in the mixture before flaming. Also, molar masses were given as following: for calcium hydroxide 74, for calcium carbonate 100 and for calcium oxide 56 g per mole. I even don't have a clue how to start.
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Offline mjc123

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Re: Mass fraction?
« Reply #1 on: January 28, 2015, 04:52:11 AM »
Can you write chemical equations for the reactions that are occurring?
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Offline Johulus

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Re: Mass fraction?
« Reply #2 on: January 28, 2015, 05:21:23 AM »
You mean am I able to do that or? Because they were not given. But, I guess it should be something like:
Ca(NO3)2(s) + Ca(OH)2(s)  :rarrow: CaO(s) + CO2(g) + Ca(OH)2(s)

But as I've said, the equation was not given. I am not really sure how to write that. I know when you flame CaCO3 that you get CaO and CO2 but what about Ca(OH)2 I have no idea.
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Offline Hunter2

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Re: Mass fraction?
« Reply #3 on: January 28, 2015, 06:18:40 AM »
Calciumhydroxide will loose water and forms also CaO. So your 60% is only CaO. The 100% is your mixture and the 40% is the mixture of water and carbon dioxide.

I would convert one equation by adding the other one. CaO is in both in .
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Offline DrCMS

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Re: Mass fraction?
« Reply #4 on: January 28, 2015, 06:23:38 AM »
Quote from: Johulus on January 28, 2015, 05:21:23 AM
You mean am I able to do that or? Because they were not given. But, I guess it should be something like:
Ca(NO3)2(s) + Ca(OH)2(s)  :rarrow: CaO(s) + CO2(g) + Ca(OH)2(s)

But as I've said, the equation was not given. I am not really sure how to write that. I know when you flame CaCO3 that you get CaO and CO2 but what about Ca(OH)2 I have no idea.

The question is asking about calcium carbonate so why have you written an equation with calcium nitrate?
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Offline Johulus

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Re: Mass fraction?
« Reply #5 on: January 28, 2015, 07:26:01 AM »
Quote from: DrCMS on January 28, 2015, 06:23:38 AM
Quote from: Johulus on January 28, 2015, 05:21:23 AM
You mean am I able to do that or? Because they were not given. But, I guess it should be something like:
Ca(NO3)2(s) + Ca(OH)2(s)  :rarrow: CaO(s) + CO2(g) + Ca(OH)2(s)

But as I've said, the equation was not given. I am not really sure how to write that. I know when you flame CaCO3 that you get CaO and CO2 but what about Ca(OH)2 I have no idea.
sorry was a mistake :/
The question is asking about calcium carbonate so why have you written an equation with calcium nitrate?
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Offline Hunter2

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Re: Mass fraction?
« Reply #6 on: January 28, 2015, 07:59:03 AM »
So how is your solution now?

Start with 100% = m% CaCO3 + m% Ca(OH)2

Develop this for the water Carbondioxide mixture as well.

Convert all to mole.

Consider n(CaCO3 = is equal to what and also n Ca(OH)2 is equal to water.

You can develop two (three) equations.

eleminate one of the variable by converting.
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Offline Borek

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Re: Mass fraction?
« Reply #7 on: January 28, 2015, 11:38:46 AM »
Quote from: Johulus on January 28, 2015, 05:21:23 AM
Ca(NO3)2(s) + Ca(OH)2(s)  :rarrow: CaO(s) + CO2(g) + Ca(OH)2(s)

Apart from the fact you used nitrate instead of carbonate, you also did another mistake - these are two separate reaction equations, not one. Each substance decomposes separately, which means each changes its mass in a different way.
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Offline Johulus

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Re: Mass fraction?
« Reply #8 on: February 01, 2015, 01:40:32 AM »
So..

CaCO3(s) :rarrow: CaO(s) + CO2(g)

and

Ca(OH)2(s)  :rarrow: CaO(s) + H2O(g)

m(CaO)=0.6 * (m(CaCO3)+m(Ca(OH)2))

Can I add upper equations one to another or not?

Ca(OH)2(s) + CaCO3(s)  :rarrow: 2CaO(s) + H2O(g) +  CO2(g)

Now how should I...
Do I need any other data except molar masses or ? What if I say that 1 mole of Ca(OH)2(s) and 1 mole of CaCO3(s) entered the equation and then....
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Offline Borek

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Re: Mass fraction?
« Reply #9 on: February 01, 2015, 03:40:57 AM »
Quote from: Johulus on February 01, 2015, 01:40:32 AM
So..

CaCO3(s) :rarrow: CaO(s) + CO2(g)

and

Ca(OH)2(s)  :rarrow: CaO(s) + H2O(g)

m(CaO)=0.6 * (m(CaCO3)+m(Ca(OH)2))

Perfect so far.

Now, you should be also able to write another, slightly similar equation, expressing mass of CaO produced through the stoichiometry of both reactions. How much CaO will be produced from m(CaCO3)?

Will you be able to solve the problem having these two equations? It is just a simple algebra then.

Quote
Can I add upper equations one to another or not?

Ca(OH)2(s) + CaCO3(s)  :rarrow: 2CaO(s) + H2O(g) +  CO2(g)

And why not

17Ca(OH)2(s) + 31CaCO3(s)  :rarrow: 48CaO(s) + 17H2O(g) + 31CO2(g)

neither of these is "better", but as you see after adding them you won't have a unique reaction equation, so you can't use it for calculations. Reactions go separately, not together (even if they occur in the same place), and each one is governed by its own stoichiometry.

Quote
Do I need any other data except molar masses or ? What if I say that 1 mole of Ca(OH)2(s) and 1 mole of CaCO3(s) entered the equation and then....

If the problem was that easy, you could answer it right away, just converting molar ratio to mass ratio.
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Offline Johulus

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Re: Mass fraction?
« Reply #10 on: February 01, 2015, 06:11:01 AM »
I think I did it. I've come across the same task somewhere else but it is said that there is 100 g of the initial mixture. It makes the whole thing easier. I guess, actually, that they had left out that mass because maybe they expected that I would use 100 g of the initial mixture while calculating.
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Offline Borek

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Re: Mass fraction?
« Reply #11 on: February 01, 2015, 11:27:47 AM »
Assuming 100g (or any other number) for the total mass makes things easier, but is not necessary - once you get both equations you can solve the system for the ratio of the masses and total mass will cancel out.
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