Molarity is represented by M and its unit is also M?

  • #1
SHASHWAT PRATAP SING
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In my textbook it is given that
Molarity is represented by M,
and its formula is--

Molarity(M)= No. of moles of solute/Volume of solution in L

the unit of Molarity is mol/l which is also written as M.
Now what does this mean ?
does it mean --> Molarity is denoted by "M" and it's unit is also denoted by "M" ? am I right ?
it's confusing,
So, for example if the Molarity of a solution is 0.4 M, so should we write like this-
M = 0.4 M
I am confused please help me.
 

Answers and Replies

  • #2
anuttarasammyak
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One mole contains exactly 6.02214076×10^23 elementary entities as well a dozen does 12, a kilo does 1,000.
Mol and Mol/litter should be distinguished as well 60 k m and 60k m/h.
 
  • #3
SHASHWAT PRATAP SING
103
7
One mole contains exactly 6.02214076×10^23 elementary entities as well a dozen does 12, a kilo does 1,000.
Mol and Mol/litter should be distinguished as well 60 k m and 60k m/h.
anuttarasammyak could please explain me in detail as I didn't understand what you said.Please...
 
  • #4
anuttarasammyak
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One mole says there are 6.02214076×10^23 molecules.
One mole/litter = M says there are 6.02214076×10^23 molecules per one litter of solution. M is molar concentration, number density of molecules.
1 litter of 1 M sugar solution contains 1 mole sugar molecules.
 
  • #5
SHASHWAT PRATAP SING
103
7
One mole says there are 6.02214076×10^23 molecules.
One mole/litter = M says there are 6.02214076×10^23 molecules per one litter of solution. M is molar concentration, number density of molecules.
1 litter of 1 M sugar solution contains 1 mole sugar molecules.
anuttarasammyak My question is this--> Molarity is denoted by "M" and it's unit is also denoted by "M" ?
I am asking about the representation and unit of Molarity. Are they both same ?
 
  • #6
anuttarasammyak
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What is molarity you mean ?
Molar concentration is denoted by M which is equal to mol/l.
Number of molecules is counted by unit mol.

1 litter of 1M solution contains 1 mol molecules we refer.
as well as
1 litter of 10g/l salt solution contains 10 g salt.
 
  • #7
Borek
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If I understand your confusion correctly, it stems from the fact the same symbol is used both for a unit (molarity) and quantity (concentration). I never use the latter, I always denote the concentration just by C.

While there is some potential ambiguity here, saying "M is measured in M" is not much different from saying "P is measured in Pa" (pressure is measured in pascals).
 
  • #8
SHASHWAT PRATAP SING
103
7
If I understand your confusion correctly, it stems from the fact the same symbol is used both for a unit (molarity) and quantity (concentration). I never use the latter, I always denote the concentration just by C.

While there is some potential ambiguity here, saying "M is measured in M" is not much different from saying "P is measured in Pa" (pressure is measured in pascals).
Borek yes you understood my confusion correctly.
Please, tell me is this correct or not that --> Molarity is denoted by M and it's unit is also denoted by M.
 
  • #9
Borek
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Please, tell me is this correct or not that --> Molarity is denoted by M and it's unit is also denoted by M.

Yes.

As I wrote: I never use M to denote quantity, I use C, so I never have that problem. But it is just a matter of notation convention, which is secondary to the concept.
 
  • #10
SHASHWAT PRATAP SING
103
7
Yes.

As I wrote: I never use M to denote quantity, I use C, so I never have that problem. But it is just a matter of notation convention, which is secondary to the concept.
Borek thanks for help,
so, if a solution is described as 4M, then we can write it as-
M = 4M
here, M on the left is Molarity and the M on the right is Molar. The unit of Molarity is mol/l which is also denoted by M and called Molar.
 
  • #11
Borek
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so, if a solution is described as 4M, then we can write it as-
M = 4M
here, M on the left is Molarity and the M on the right is Molar.

English being my second language I am not 100% sure "Molar" is the right use of the word here.

But in general yes, you are right about the meanings. As I wrote earlier I prefer C=4M, which is much less ambiguous.

The unit of Molarity is mol/l which is also denoted by M and called Molar.

Same problem as above. We need to wait for someone with a better grip of the language.
 
  • #12
SHASHWAT PRATAP SING
103
7
Borek, am I right here-
so, if a solution is described as 4M, then we can write it as-
M = 4M
here, M on the left is Molarity and M on the right represents the unit of Molarity which is mol/l .
 
  • #13
DrStupid
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so, if a solution is described as 4M, then we can write it as-
M = 4M
here, M on the left is Molarity and M on the right represents the unit of Molarity which is mol/l .

Using the same symbol with different meanings in the same equation is a bad idea. Better replace one of the Ms by something different, e.g. C = 4 M (as suggested by @Borek) or M = 4 mol/l.
 
  • #14
Mayhem
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Molarity, also known as molar concentration, is often represented by the letter c (as in c = n/V) and has units mol/L or M (they are completely equivalent). Molar mass is represented by the letter M (as in m = nM) and is measured in g/mol.
 
  • #16
SHASHWAT PRATAP SING
103
7
https://goldbook.iupac.org/terms/view/A00295 says "sometimes denoted by M" - so no, mol/L and M are not "completely equivalent".
Borek,now what does this mean.

you said "Yes" when i asked --> Molarity is denoted by M and it's unit is also denoted by M ?

then now you are saying "sometimes denoted by M" - so no, mol/L and M are not "completely equivalent".

what does this mean ?
 
  • #17
anuttarasammyak
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Wikipedia https://en.wikipedia.org/wiki/Molar_concentration shows us typical examples of usage of the symbols, e.g.,

"For example, if a sodium carbonate solution (Na2CO3) has a formal concentration of c(Na2CO3) = 1 mol/L, the molar concentrations are c(Na+) = 2 mol/L and c(CO3 2-) = 1 mol/L"

  • 11.6 g of NaCl is dissolved in 100 g of water. The final mass concentration ρ(NaCl) isρ(NaCl) = 11.6 g/11.6 g + 100 g = 0.104 g/g = 10.4 %.
    The density of such a solution is 1.07 g/mL, thus its volume is
    V= 11.6 g + 100 g/1.07 g/mL = 104.3 mL.
    The molar concentration of NaCl in the solution is therefore
    c(NaCl) = 11.6 g/58 g/mol / 104.3 mL = 0.00192 mol/mL = 1.92 mol/L.Here, 58 g/mol is the molar mass of NaCl.
  • A typical task in chemistry is the preparation of 100 mL (= 0.1 L) of a 2 mol/L solution of NaCl in water. The mass of salt needed ism(NaCl) = 2 mol/L × 0.1 L × 58 g/mol = 11.6 g.To create the solution, 11.6 g NaCl is placed in a volumetric flask, dissolved in some water, then followed by the addition of more water until the total volume reaches 100 mL.
  • The density of water is approximately 1000 g/L and its molar mass is 18.02 g/mol (or 1/18.02 = 0.055 mol/g). Therefore, the molar concentration of water isc(H2O) = 1000 g/L/18.02 g/mol ≈ 55.5 mol/L.Likewise, the concentration of solid hydrogen (molar mass = 2.02 g/mol) isc(H2) = 88 g/L/2.02 g/mol = 43.7 mol/L.The concentration of pure osmium tetroxide (molar mass = 254.23 g/mol) isc(OsO4) = 5.1 kg/L/254.23 g/mol = 20.1 mol/L.
  • A typical protein in bacteria, such as E. coli, may have about 60 copies, and the volume of a bacterium is about 10−15 L. Thus, the number concentration C isC = 60 / (10−15 L)= 6×1016 L−1.The molar concentration isc = C/NA = 6×1016 L−1/6×1023 mol−1 = 10−7 mol/L = 100 nmol/L

My personal recommendation to you is to stop using letter M for any meanings. You see M does not appear in the above examples. M is just traditional so unofficial unit not included in SI unit. And as you worry about careless students might observe
[tex]M=4M[/tex]
as mathematics equation and answer "M=0".
 
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  • #18
Mayhem
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  • #19
Borek
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They don't define it this way, they say "people tend to to use M to denote mol/L even if we never suggested it". What do you expect them to do? Force people to follow the definition at a gun point?
 
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  • #20
Vanadium 50
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What do you expect them to do? Force people to follow the definition at a gun point?

I see a new prime-time TV police procedural: "IUPAC: Enforcement"
 
  • #21
f95toli
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They don't define it this way, they say "people tend to to use M to denote mol/L even if we never suggested it". What do you expect them to do? Force people to follow the definition at a gun point?
I would assume it depends on the context, or? In most regulatory/standards contexts you HAVE to use SI units to be compliant and these situations can't be that unusual chemistry, or?
I would have assumed that e.g. a pharmaceutical company or even a chemical process plant would be in trouble if they were audited and found to be using "weird" units/notation for their internal documentation(!)
 
  • #22
Borek
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mol/L is an unambiguous SI unit, the only potentially ambiguous thing is the convention of using M for that. Probably the best approach is to precisely define notation used in the particular document, as long as it is clear and not unnecessarily confusing I don't think it matters much whether one writes M or mol/L.

Definitely better to have a well defined standard used throughout the company or even throughout the world. Among other things that would save Mars Climate Orbiter :wink:
 
  • #23
Ygggdrasil
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Why is is problematic for the quantity and its unit to be represented by the same letter? This occurs in other situations, such as voltage being represented by V (e.g. Ohm's law V = IR) and its unit being Volt (V).
 
  • #24
vela
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Also, when the math is typeset, variables are typically denoted using italics while units are not, e.g., ##M = 4~\rm M##. If you're writing it out by hand, it's probably best to do as Borek does and use a different symbol for the concentration or use mol/L instead of M for the units.
 

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