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Topic: Percentage of hydrates  (Read 6351 times)

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MakoEyes

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Percentage of hydrates
« on: November 02, 2009, 01:33:21 PM »
Hey I got a question:

A solid hydrate weighing 2.4 g was heated to drive off the water. A solid anhydrous residue remained,
which weighed in at 1.2 g.

Calculate the % water in the hydrate and if the anhydrous residue has a formula weight of 132, how many moles of water are present in one mole of hydrate?

Can someone show me step by step how to approach this problem?

CIE alevel

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Re: Percentage of hydrates
« Reply #1 on: November 02, 2009, 02:33:12 PM »
when hydrated salt is heated what does it decompose into?

u know mass of hydrated salt heated and u know mass of anhydrous salt after heating...
So how can u find mass of water from this? What does anhydrous mean?

Once u got the mass of water...how can u find the % of water & moles of water in hydrated salt?

Anyways check this out: http://chem.lapeer.org/Chem1Docs/HydrateLab1.html

MakoEyes

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Re: Percentage of hydrates
« Reply #2 on: November 03, 2009, 09:23:40 PM »
when hydrated salt is heated what does it decompose into?

u know mass of hydrated salt heated and u know mass of anhydrous salt after heating...
So how can u find mass of water from this? What does anhydrous mean?

Once u got the mass of water...how can u find the % of water & moles of water in hydrated salt?

Anyways check this out: http://chem.lapeer.org/Chem1Docs/HydrateLab1.html

So can I write...

2.4g-1.2g = 1.2g

1.2g H2O / 2.4g sample * 100% = answer ?

I'm still a bit confused on the moles...

MakoEyes

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Re: Percentage of hydrates
« Reply #3 on: November 03, 2009, 10:45:21 PM »
Ah sorry I looked a little deeper into this...

% of water in hydrate = ( (2.4 - 1.2)/2.4) x 100 = 50%

1.2g anhydrate * (1 mole H2O / 18.016g H2O) = 0.067 mole H2O

1.2g Anhydrate * (1 mole anhydrate / 132g anhydrate) = 0.009090 mole anhydrate

I am unsure of where to go from here, any help is appreciated.

Borek

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Re: Percentage of hydrates
« Reply #4 on: November 04, 2009, 02:54:28 AM »
What is ratio of number of moles of water and salt? (In other words: how many moles of water per mole of salt?)
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info, PZWT_s1

MakoEyes

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Re: Percentage of hydrates
« Reply #5 on: November 04, 2009, 12:47:28 PM »
What is ratio of number of moles of water and salt? (In other words: how many moles of water per mole of salt?)

0.067 mole H2O :: 0.009090 mole anhydrate salt

I don't know what to do with this value though...

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