Sciencemadness Discussion Board

Relative number of moles

A.K. - 1-12-2023 at 02:15

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

Fulmen - 1-12-2023 at 05:35

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

Maurice VD 37 - 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.

Fulmen - 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?

DraconicAcid - 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.

B(a)P - 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]

Fulmen - 1-12-2023 at 11:30

Why not cuprous peroxide? :-)

DraconicAcid - 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. :-)

j_sum1 - 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.

Moles

MadHatter - 2-12-2023 at 11:02

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.

DraconicAcid - 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.

A.K. - 31-12-2023 at 23:32

Thanks everyone for their great effort to answer my question.

Maurice VD 37 - 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]

woelen - 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.

woelen - 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).

A.K. - 21-6-2024 at 13:31

Again, Thanks everyone for their efforts to explain this term "Relative number"
After a longer period of time, I find out what does "Relative number of moles" means. It is not related to the other element within a compound, That is wrong!
Majority of the answers got it wrong with all due respect.
only "Maurice VD 37" know what he is talking about,

Thanks

DraconicAcid - 21-6-2024 at 13:40

Quote: Originally posted by A.K.  
Again, Thanks everyone for their efforts to explain this term "Relative number"
After a longer period of time, I find out what does "Relative number of moles" means. It is not related to the other element within a compound, That is wrong!


So when the author of the question you were struggling with said "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", he didn't know what he was talking about?

A.K. - 21-6-2024 at 18:03

The author know what he is talking about, but you don't. You and many like you misunderstood the meaning of "Relative number of moles".
Since the compound that we have in term of empirical formula is in percentage, and we don't have an actual mass for it, The author used the term "Relative number of moles" Instead of "Actual or absolute number of moles"
Check this website will clarify the doubt if you are truly seeking the Truth. Otherwise ignore it!

https://www.dr-aart.nl/Statistics-absolute-and-relative.html...

DraconicAcid - 21-6-2024 at 19:14

It's too bad you weren't able to comprehend what we explained to you. Welcome to the block bin.

bnull - 22-6-2024 at 03:53

Quote:
if you are truly seeking the Truth.

What is truth? said Pontius Pilate.

McMurry and Fay gave the explanation away for free within the same section. If you revisit the section carefully, you will see that. Nevertheless, I'll add my five kopeks to this discussion.

Quote: Originally posted by A.K.  
"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"

Let's quote the paragraph in full:
Quote:
Knowing a compound's percent composition makes it possible to calculate the compound's chemical formula. As shown in Figure 3.8 [which I won't include because it is just a flow diagram], 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. The mole ratios, in turn, give the subscripts in the chemical formula.

We have some important points. The first is that purpose of section 3.11 is to show how to find the chemical formula for the compound starting from the percent composition. It... sort of works... The main issue is that it doesn't take into account the bonds between elements. Acetylene and benzene have the exact same percent composition, but acetylene is a gas and benzene is a liquid with different properties. That's why it is possible. It gives the empirical formula, which is not always equal to the actual chemical (molecular or ionic) formula.

The second is what did McMurry and Fay mean with "relative number of moles of each element in the compound"? The answer is simple: that there are X moles of A for each Y moles of B, whatever the mass of the sample may be. When you say
Quote:
Since the compound that we have in term of empirical formula is in percentage, and we don't have an actual mass for it
you forget that the paragraph that follows the one cited above goes
Quote:
Let's use for our example a colorless liquid whose composition is 84.1% carbon and 15.9% hydrogen by mass. Arbitrarily taking 100 g of the substance to make the calculation easier, we find by using molar masses as conversion factors that the 100 g contains 7.00 mol of C and 15.8 mol of H.
The percent composition means that in 100 grams of the substance there are 84.1 grams of carbon and 15.9 grams of hydrogen. The actual mass of the sample is immaterial.

Take astatine fluoride, my favorite fluorinating agent. There are 91.7 grams of astatine for each 8.3 grams of fluorine in 100 grams of AtF. But there are no 91.7 grams of astatine on Earth because it is unstable. Does it matter? No, because we're interested in the empirical formula, which does not require the existence of 100 grams of AtF, or of 91.7 grams of astatine itself, for that matter.
Quote:
Question 1. What does is the definition of Relative number in chemistry?

Question 1 was answered more times than it really deserved so I shall skip it.
Quote:
Question 2. Why did the author didn't say the actual number of moles instead of relative number of moles?

Because the actual number of moles has no importance whatsoever when you're trying to find the empirical formula. Water is always H2O whether you have 100 grams, 25 micrograms, or the entire water supply of the known universe. The proportion remains the same, and so does the empirical formula. The actual number of moles comes into play when you want to perform a reaction using a definite amount of the compound (stoichiometry and such).
Quote:
Question 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?

To one another within the compound. If you want to find the empirical formula of one compound (say, water), you relate the number of moles of the elements that constitute the compound. You can't find the empirical formula for water by comparing with the amount of carbon in benzene: "There are two moles of hydrogen in water for each six moles of carbon in benzene". It doesn't work that way. What @Maurice VD 37 explained was concentration in terms of percentages and proportions. Section 3.11 deals with percent composition and empirical formulas of a single compound, not concentrations. You see, apples and oranges.

And you are right. McMurry and Fay knew what they were talking about, but you don't. You misunderstood the meaning of "Relative number of moles".

Edit: corrected typos (again) and reworded some parts. Who cares?

[Edited on 22-6-2024 by bnull]

[Edited on 23-6-2024 by bnull]