1.3 Formulae, equations and amounts of substance
Posted by: intechemistry on: September 23, 2010
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8 Responses to "1.3 Formulae, equations and amounts of substance"
May 25, 2011 at 12:10 am
Good day to you too, sinhui (10M9)
To me, in some ways, ‘atom economy’ is a rather ‘made-up’ concept, i.e. it’s not what I’d call a ‘scientific/physical’ parameter.
As far as I am aware, the typical way it’s calculated, is based on mass, not numbers of atoms, although I don’t see why a measure of atom economy couldn’t be done by number of atoms.
I’d stick with mass.
My guess is that if you showed working out using number of atoms instead (and calculated the right answer) the examiner would also score that.
May 10, 2012 at 5:58 pm
Salam alaik Sir. 
I have a q about Double Salt. In the AS Pearson book page 30, at the bottom of the page, it stated that “Double salts are crystals that contain two different salts in a 1:1 ratio.”
The stated example there is KAl(SO4).12H20. So, i dont really get the point, how it can get the 1:1 ratio? Is it based on molar mass ratio?
TQ Sir for reading and answering.
May 10, 2012 at 11:43 pm
Waalaikumassalam Zen
Ah, you’ve exposed my weakness… AS info 
I’ve never considered double salts significant, hence never bothered to have anything to do with them (as it wasn’t within the stuff I taught)
A quick ‘Google’ search gives this, which I think is quite informative.
1) Double salt – Wikipedia, the free encyclopedia
en.wikipedia.org/wiki/Double_salt
Double salts are salts containing more than one cation or anion. They form when more than one salt is dissolved in a liquid and when together they crystallize in …
2) double salt – definition of double salt by the Free Online Dictionary …
http://www.thefreedictionary.com/double+salt
double salt. n. A salt, such as alum, that ionizes in solution as if it were two separate salts but forms a single substance upon crystallization.
3) Making Double Salts
chem.lapeer.org/Chem1Docs/DoubleSalt.html
A “double salt”has two different cations in their crystal lattice, in a definite proportion. Double salts commonly form large crystals more readily than do the single …
4) Double salt | Define Double salt at Dictionary.com
dictionary.reference.com/browse/double+salt
double salt. noun Chemistry. a salt that crystallizes as a single substance but ionizes as two distinct salts when dissolved, as carnallite, KMgCl3⋅6H2O.
——————–
I’ve not heard of any necessity for a 1:1 ratio though. Sounds quite peculiar to me. The Edexcel revision guide says roughly what those 4 search results say and nothing about 1:1 ratios. It does say when driving off the water (to crystallize out the salt from the solution) don’t evaporate the solution to dryness by heating it as the crystals may decompose.
I think the 1:1 thing is maybe what EDEXCEL WANT TO EXPOSE YOU TO IN THIS SPECIFICATION as certain examples of double salts (although that’s just a pure guess).
Lets consider aluminium sulfate. Take one mole of it. Formula = Al2(SO4)3).
Now lets take one mole of potassium sulfate = K2SO4. Add these together gives: Al2K2(SO4)4 {perhaps do the same for the waters of crystallization ???}
The formula can then be divided by two to get the simple whole number ratio, giving AlK(SO4)2 or KAl(SO4)2.
My answer differs from yours. You only have one SO4. Did you make a typo? because I cant see how a formula could be KAl(SO4).12H20 as that species would have a 2- charge hence isn’t a ‘complete’ formula.
May 11, 2012 at 12:13 am
Oppss, sorry Sir.
There’s typo, it should be KAl.(SO4)2.12H2O. But not (SO4)4. Hmm, I just would like to know as they ask a bit about double salt in the Unit 3B paper the other day. Just afraid if i just learn double salt based on mark scheme on past years papers but not well understand it.
But, what’s the relationship of the salts to the water?
May 11, 2012 at 12:37 am
Humm… One of my paragraphs went walkabout. I’ve corrected it above.
‘But, what’s the relationship of the salts to the water?’ – Could you elaborate? Do you mean the water of crystallization in the formula of the salt?
May 11, 2012 at 12:58 am
“Do you mean the water of crystallization in the formula of the salt?” – yes, Sir. sorry for being vague.
May 11, 2012 at 1:24 am
Water of crystallization is the water(molecules) that exists in a regular crystal structure and facilitates/contributes towards the unit cell of the salt, i.e. the smallest representative arrangement making up the whole of the crystal of the compound.
I don’t think the numbers in water of crystallization can be predicted, and that’s why (I’m guessing here) you are required to be able to calculate the ratio of water involved.
If the water isn’t there (or id driven off by heat), the crystal structure breaks down and an amorphous (i.e. non-crystalline) powder usually results [which is often white].
You may like to read this: http://intechemistry.wordpress.com/2012/01/29/water-of-crystallisation/
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May 24, 2011 at 8:54 pm
Good day Sir,
I would like to ask if atom economy is the sum of the atomic mass of desired product over reactant (as most books said that) or the sum of number of atoms in the desired product over reactant (George Facer)?
Thanks.