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A.K.
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[*] posted on 1-12-2023 at 02:15
Relative number of moles


Hello Everyone!
I am new here.
I am a Mech. Eng, and I do self studies in Chemistry during my spare time.
I am reading a Chemistry book By Sir. McMurry and Fay. During chapter 3 page97, topic 3.11 percent composition, I have encountered a term that I didn't quite get it yet even after several researches on google and in the library so here is a quote from the author:
"the strategy is to find the relative number of moles of each element in the compound and then use the numbers to establish the mole ratios of the elements"
My first question is:
1. What does is the definition of Relative number in chemistry?
2. Why did the author didn't say the actual number of moles instead of relative number of moles?
3. If we are relating the number of moles, to whom are we relating to? are we relating it to the other element in the compound that we have just discovered itself? or we are relating it to another element from a different chemical compound?
Thanks
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Fulmen
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[*] posted on 1-12-2023 at 05:35


Example: Al2O3 has a relative ratio of 1:1.5 but an absolute molar ratio of 2:3.



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Maurice VD 37
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[*] posted on 1-12-2023 at 07:31


Let's take an example.
1 liter sea water contains 29 g NaCl 0r 29/(23 + 35.5) = 0.49 mol NaCl.
1 liter sea water contains also 55.5 mol water
So 1 liter sea water contains a total of 56 mol.
The relative amount of NaCl in water is 0.49/56 = 0.875 percent
This is a relative amount. it is valid for one drop or 1 ton sea water.
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[*] posted on 1-12-2023 at 09:32


To elaborate a bit:
Lets say you have an unknown gas. You do a chemical analysis which tell you it contains 3 moles of hydrogen for every mole of carbon. Does this mean it's composition is CH3?




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DraconicAcid
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[*] posted on 1-12-2023 at 11:24


Quote: Originally posted by A.K.  

I am reading a Chemistry book By Sir. McMurry and Fay. During chapter 3 page97, topic 3.11 percent composition, I have encountered a term that I didn't quite get it yet even after several researches on google and in the library so here is a quote from the author:
"the strategy is to find the relative number of moles of each element in the compound and then use the numbers to establish the mole ratios of the elements"
My first question is:
1. What does is the definition of Relative number in chemistry?
2. Why did the author didn't say the actual number of moles instead of relative number of moles?
3. If we are relating the number of moles, to whom are we relating to? are we relating it to the other element in the compound that we have just discovered itself? or we are relating it to another element from a different chemical compound?
Thanks


It's number of moles of an element relative to the other elements in the compound.

If you have a compound that is, say, 79.9% copper and 20.1% oxygen, you're not going to worry about an actual number of moles, since you don't have an actual sample to worry about. But you do know you have 79.9 g Cu for every 20.1 g O. Converting these to moles gives you 1.26 mole Cu for every 1.26 moles O. That's a 1:1 ratio, so it's CuO.




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[*] posted on 1-12-2023 at 11:30


Is it not simpler than what had been suggested above? I am guessing they don't give you a mass of the compound? If you don't have a mass of a compound you can't work out how many moles of each element in the compound. What you can work out is the relative number of moles. For H20 the relative number of moles of hydrogen is 2. It is a simple, but important concept to master, the next step is to work out the actual mass of hydrogen in a known mass of water, now that you know how to work out the relative number of moles.

Edit - I like DraconicAcid's answer better I had not seen it when I posted. However, the answer to your question depends on what information you have been given in the question, please provide am an example question from the text if one is available.
Oh, and importantly, welcome to the forum!

[Edited on 2-12-2023 by B(a)P]
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[*] posted on 1-12-2023 at 11:30


Why not cuprous peroxide? :-)



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[*] posted on 1-12-2023 at 11:34


Quote: Originally posted by Fulmen  
Why not cuprous peroxide? :-)

Well, the calculation gives the empirical formula, which is just the simplest ratio. We wouldn't know for sure if it was CuO or Cu2O2 or Cu7O7 without knowing the molar mass of the compound.

Other than knowing that peroxide ion would immediately oxidize cuprous ions to cupric ones. :-)




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[*] posted on 1-12-2023 at 15:07


I will add that there are ways of teaching moles that are really confusing. Poorly phrased questions such as this one do not help.

As a teacher, I know that I have to give careful thought to how I introduce mole calculations to my students. It is like the foundation of a building: get it wrong and everything that follows is unstable and crooked.

If this question was in one of my textbooks, I would have two approaches. One is to skip it completely. The second would be to consider the information orivided and discuss with students what sensible calculations could be done - and then do them; pretty much ignoring whether we were interpreting the question the way the author intended. The chemisrty is important. Ticking the box to say you got the right answer to a poorly constructed question is not.
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[*] posted on 2-12-2023 at 11:02
Moles


j_sum1, I'll go with most on "moles". A mole is a mole is a mole. I don't about relative
but absolute. Thus Al2O3 is 2:3. Don't know anything relative about that. The OP's
question seems irrelevant.




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[*] posted on 2-12-2023 at 15:17


Quote: Originally posted by MadHatter  
j_sum1, I'll go with most on "moles". A mole is a mole is a mole. I don't about relative
but absolute. Thus Al2O3 is 2:3. Don't know anything relative about that. The OP's
question seems irrelevant.


Of course that's relative. It's the moles of Al relative to the number of moles of O. If you have any given sample of alumina, you do not necessarily have 2 mol Al and 3 mol O. It depends on how much you have.




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[*] posted on 31-12-2023 at 23:32


Thanks everyone for their great effort to answer my question.
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Maurice VD 37
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[*] posted on 5-1-2024 at 09:47


I repeat my reasoning with another example : blood instead of sea water. What is the relative amount of salt in the plasma, which is blood without red cells ?
In solutions, the relative amount is another word for describing the concentration.
Well, one liter blood or 1 liter plasma contains 9 g NaCl. One can say that the concentration or the relative amount (in mass) of NaCl in plasma is 9 g/liter. And the problem is solved, because this result is the same for one drop or for many liters plasma.
I repeat. Expressed ion g/L, the relative amount of NaCl is 9 g/L, whatever the volume of blood and plasma.
But the relative amount can also be expressed in molar units. It depends on the problem.
As the molar mass of NaCl is 23 + 35.5 = 58.5 g/mole. The relative amount can be calculated with the moles. Expressed in moles, 9 g NaCl is also 9/58.5 = 0.154 mole, and the relative amount of salt in blood is 0.154 mol/Liter.
Conclusion : The relative amount of salt in blood is 0.9 g/L, or 0.154 mol/L, as you want it.

[Edited on 5-1-2024 by Maurice VD 37]

[Edited on 5-1-2024 by Maurice VD 37]
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[*] posted on 8-1-2024 at 00:46


Quote: Originally posted by Fulmen  
Example: Al2O3 has a relative ratio of 1:1.5 but an absolute molar ratio of 2:3.

The usage of the terms "relative"and "absolute" is meaningless here. Al2O3 has aluminium and oxygen in a molar ration equal to 1 : 1.5, which is the same as 2 : 3, which is the same as 20 : 30, which is the same as 7 : 10.5, etc. All of them are the same, the only thing which differs is their presentation.

In practice, if there is a simple integer ratio for a given compound, then this is presented (i.e. 2 : 3), but sometimes other presentations are used.

Molar ratios do not always have to be integer values. For instance, I have tungsten oxide, WO3, which is a yellow powder (looks very much like powdered sulfur). But I also have a compound, which is somewhat deficient in oxygen, which is dark blue. It has molar ratio, close to 1 : 2.9. But the exact formula of this material cannot be given. One could write W10O29, but this formula is meaningless from a structural point of view. The real nature of this compound is that it is WO3, with a small fraction of oxygen atoms missing (appr. 1 out of every 30 oxygen atoms is missing). This small amount of missing oxygen atoms leads to a very different compound (it has an intense color, and its electrical properties are quite different from those of WO3).

If you have mixtures, then molar ratios certainly are useful for describing these mixtures, but again, no particular formula should be given and it makes no sense to try to write the mix with integer-only formulas. A nice example is azeotropic nitric acid, which is appr. 68% by weight HNO3, the rest being water. This is a molar ratio of appr. 1 : 1.65 nitric acid and water. The azeotropic mix, however, has no nice integer ratio of nitric acid and water.




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[*] posted on 8-1-2024 at 07:57


This of course occurs much more. The simplest example is H2O2. It contains H : O = 1 : 1, but the molecule definitely contains two hydrogens and two oxygens (H-O-O-H). Another example is ethane, H3C-CH3. C; H = 1 :3, but it is C2H6.

Based on ratio alone, one cannot determine the real number of atoms in a molecule. Sometimes, one cannot even speak of a simple molecule. See my previous example of W : O approximately equal to 1 : 2.9 (but not exactly so, different samples of this dark blue compound may have slightly different ratios, the ratio can e.g. be between 1 : 2.89 and 1 : 2.91). There are many compounds, which have a somewhat variable stoichiometry (e.g. transition metal oxides with mixed oxidation states for the metal ion, transition metal sulfides with mixed oxidation states for the metal ion, polymeric species, with variations in which atoms are bonded to each other, sub-oxides of cesium and rubidium).




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