# Moles, Molecular mass, Number of Molecules Im lost.

1. Sep 20, 2015

### MidgetDwarf

So I am taking the second semester of intro physics and Moles,Molecular mass, Number of Molecules is killing me.

I am not really able to solve my homework because I can never get these quantities down. I asked my professor for help during offie hours, however he said he teaches physics not chemistry. I have never had a chemistry.

This is what I understand so far.

N=nNa

where N=number of molecules, n=number of moles in substance, and Na is Alvogadros constant.

I know that if I look at a periodic table, I can find the mass (is this molecular mass/atomic mass)?

Say I want the molecular mass of Helium. From the Periodic Table i see the atomic mass 1.00794u.

I take this atomic mass and multyply by 1.66x10^-23 and I get the molecular mass in kilograms.

How do I find the moles?

2. Sep 20, 2015

### SteamKing

Staff Emeritus
If you are trying to find the molecular mass of helium from a periodic table and you come up with 1.00794u, either your periodic table is wrong or you are not using it correctly.

Below is a typical periodic table. Care to try again?

3. Sep 20, 2015

### Staff: Mentor

Moles of what?

In general, mole is just an overgrown doze, NA objects (instead of 12).

4. Sep 20, 2015

### statefxn

We need to be discussing a given mass of a compound or an element . then division of the given mass by the gram atomic weight or the gram molecular weight will yield the number of gram atoms in the case of the element or the number of moles in the case of the compound.

Last edited: Sep 20, 2015
5. Sep 20, 2015

### Staff: Mentor

In both cases you will find number of moles - either of an element, or of a compound.

Perhaps that part that confuses you are the "gram atomic weight", "gram molecular weight" and "gram atom" - don't use them. There are molar masses (ie mass of one mole) of element and compound - and then converting given mass of a substance to number of moles is trivial. Molar mass of an element is the number listed in the periodic table (yes, it has other meanings as well, don't bother about them for now), molar mass of a compound is a sum of molar masses of all atoms in the compound (which trivially follows from the fact mole of a substance contains a mole of each atom listed in its formula; note that O2 means we have listed two separate oxygen atoms, not one).

6. Sep 20, 2015

### Dr. Manoj

The mass you get from periodic table is atomic mass of an element. Molecular mass means the total mass of a given molecule for ex, H2O
you will get atomic mass of H and O from periodic table. Using it you can find molecular mass.
If you consider 22.7 litre(at S. T. P. ) of molecule it is said to be one mole of same molecule, say methane. Now it means that, there are 6.022*10^23 particles in methane. You can find moles of a given molecule if, you know grams of molecule. The formula is, No. of moles = mass of given molecule in grams / molecular mass of the same molecule.
I hope you got me. If doubts, ask.

Last edited: Sep 20, 2015
7. Sep 20, 2015

### Staff: Mentor

Pure nonsense. There is no such thing as a "litre of molecule", and it is not guaranteed that 1 liter of anything is 1 mole. May happen under some specific conditions, but you would need to list them.

Next phrase is not better.

8. Sep 20, 2015

### statefxn

the number beneath the symbol in the periodic table of the elements is the atomic mass in atomic mass units. One atomic mass unit is 1/12 the mass of a carbon 12 atom.
The mass of a carbon 12 atom is accordingly : 12.011 gram per mole/ 6.023*10^23 atom per mole ==> (12.011/6.023 )*10^-23 gram per atom. That is 1.99*10^-23 gram per atom of carbon. Thus one amu is equal to 1/12 of this quantity, namely 1.99/12*(1*10^23) = 1.66*10^-24 grams per amu.

9. Sep 20, 2015

### statefxn

A word about 22.4liters. This is the volume that is occupied by a mole of an ideal gas at standard temperature (298 deg K) and standard pressure ( 760 mm Hg). It is the standard molar volume and it contains Avogadros number of molecules

10. Sep 20, 2015

### statefxn

These are fundamental chemical concepts and they are always extensively elaborated in the initial chapters of general chemistry text.
General Chemistry is a pre-requisite for General Physics in most curricula.

11. Sep 20, 2015

### Staff: Mentor

No. 12.011 g is mass of a mole of naturally occurring carbon, not of C-12. By definition 1 mole of C-12 is exactly 12 grams (finitely many zeros after the decimal point).

12. Sep 20, 2015

### statefxn

Alright ! But the definition of the atomic mass unit is still 1.66 time ten to the negative 24th power gram per amu.

13. Sep 20, 2015

### statefxn

Some of these terms can be confusing , no doubt.
Electing to avoid confusing terminology is a reasonable pedagogic technique as long it does not lead to permanent mis-conceptualization.

14. Sep 21, 2015

### Staff: Mentor

More precisely $\frac {1} {N_A}$ (units omitted to not make it more confusing). 1.66 before the exponential part is only an approximation, and it appears to be correct in your calculations based on mass of C only because 12 and 12.011 are close enough.

15. Sep 28, 2015

### James Pelezo

Minor correction, The conditions for the standard 'Molar Volume' is 273K & 1 Atm giving 22.4 L/mole. At 298K the molar volume is 24.5 L/mol. (Charles Law)
V25C/1 Atm = 22.4L/mole(298K/273K) = 24.5 L/mol.
I find it easier to comprehend if you'll think of a 'MOLE' as the mass of substance (elements or compounds) that contains 6.02 x 1023particles. This is referred to frequently as molar mass and is equal to the formula weight of elements and molecules and expressed as gram-formula weight, or simply gms. To convert mass of substance given to moles, divide amount given by formula weight of substance. Think how many formula weights are there in the given mass. Example: Given 25 gms of NaCl (formula wt = 58 gms/mol). Convert to moles = (mass given(gms) / formula wt of compound(gms/mol) = [(25 gms) / (58 gms/mole)] =0.43 mole NaCl. Conversely, to convert from moles to mass in grams, multiply by formula weight. Given 2.865 moles of pure silver, how many grams of silver does this represent? Grams Silver (Ago ) = (moles given) x (formula weight) = 2.865 moles (107.87 gms/mol) = 309.1 grams Silver. (~\$145 in today's market.)

given grams => need moles => multiply gms by formula weight (gms/mole)
given moles => need grams => divide moles by formula weight (gms/mole)

In physics, this is found/applied extensively in the Gas Laws under Ideal Gas Law, PV =nRT, where n = moles of gas, and the gas constant is in units of L-Atm/mole-K. Since units must be similar then conversion to moles is critical as the R-value units contain 'moles'. If given mass, you must convert to moles. Divide given mass by formula weight of substance of interest. Then apply to Ideal Gas Law.

PS: Oh, I teach both chemistry and physics. I'll answer questions on both disciplines.

16. Sep 28, 2015

### boonebr

This is all dimensional analysis stuff and is taught in chapter one in any physics book. Mol is just a unit of measurement like a dozen. Except 1 mol = 6x10/\23 particles

17. Sep 28, 2015

### James Pelezo

Correction on note: given grams => need moles => DIVIDE by formula weight
given moles => need grams => MULTIPLY by formula weight
Sorry bout that.

18. Oct 3, 2015

### James Pelezo

1 mole is the mass of substance containing 6.02 x 1023 particles. 1 mole = 1 Avogadros No of particles = 1 formula weight of substance. Substance can be elements or compounds. 1 mole of Carbon Atoms = 12.0111 grams which contains 6.02 X 1023 Carbon Atoms. 1 mole of water (H2O) = 18 grams which contains 6.02 x 1023 molecules of water and 2(6.02 x 1023 atoms of Hydrogen and 1(6.02 x 1023) atoms of Oxygen; or 1 mole of water molecules has 2 moles of Hydrogen and 1 mole of Oxygen atoms.

The mole is the quintessential concept in applying chemical stoichiometry to balanced chemical equations. Yes, the mole can be used via dimensional analysis, but - in my humble opinion - far more important than merely referring to it as just 'dimensional analysis stuff'. The quantities ( gms, particle numbers, gas volumes, solution reactions concentrations and energy values) can be calculated from a balanced chemical equations because the quantities can be linked to the coefficients (mole ratios) of balanced equations. Referring to the mole as just dimensional analysis stuff is like saying a diamond is equal in value to glass. Chemistry is reduced to its simplest empirical form through the mole concept and applied mathematically more often than any fundamental concept in chemistry. It is to be worshiped as the God of Chemistry!! and nothing less.

19. Oct 4, 2015

### Staff: Mentor

No. By IUPAC definition it is amount of the substance that contains NA particles. There is nothing wrong with having a mole of massless photons in which case mass is exactly zero.

20. Oct 4, 2015

### James Pelezo

I agree 100%, but when trying to create a tangible image around the definition for instructional purposes relating to moles, the 'mass' containing No = formula wt. seems to click...well, for me I get positive outcomes... Anyway, I later use the more abstract when the students get grounded and can applying the concept more easily. I used the 'zero' mass concept when discussing radioactivity, but apply it to beta emissions; that is ... What is the weight change occurring in one mole of C-14 atoms which decays by beta emission to N-14 after losing 1 mole of electrons? We use published particle rest mass values (mass defect not considered) and calculate the weight change expected for 1 Avogadro's Number of C-14 atoms. To see a difference, we have to carry the mass changes (theoretically) out to ~ 10-5gram, which simply means the 1 mole of electrons (or, beta particles) emitted have essentially 0 weight. Very interesting calculation, and the kids get a kick out of the results. Oh yeah, the loss of 1 mole of e-'s represents the loss of ~5 x 107Kj energy using the Einstein equation ∆E =Nomc2. Sure glad it's rupture rate is on a nano level ∆E.