December 09, 2019, 09:39:04 PM
Forum Rules: Read This Before Posting


Topic: Percentage of hydrates  (Read 7073 times)

0 Members and 1 Guest are viewing this topic.

Offline MakoEyes

  • Regular Member
  • ***
  • Posts: 27
  • Mole Snacks: +0/-0
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?

Offline CIE alevel

  • Regular Member
  • ***
  • Posts: 32
  • Mole Snacks: +0/-1
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

Offline MakoEyes

  • Regular Member
  • ***
  • Posts: 27
  • Mole Snacks: +0/-0
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...

Offline MakoEyes

  • Regular Member
  • ***
  • Posts: 27
  • Mole Snacks: +0/-0
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.

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 25360
  • Mole Snacks: +1663/-398
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
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

Offline MakoEyes

  • Regular Member
  • ***
  • Posts: 27
  • Mole Snacks: +0/-0
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...

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 25360
  • Mole Snacks: +1663/-398
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Percentage of hydrates
« Reply #6 on: November 04, 2009, 01:30:58 PM »
I hate repeating myself, but I will try once again. How many moles of water per 1 mole of salt?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Sponsored Links