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Why mole and kelvin are basic units? 
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#1
Dec312, 07:06 AM

P: 260

Mole is just a number. It doesn't really measure anything so why is it a fundamental unit?
And with kelvin  it represents the average energy of the atoms/molecules in a compound. Why is it a fundamental unit? Temperature can easily be represented in joules. 


#2
Dec312, 07:21 AM

P: 2,928

So,moe of the mole usage, history units of measure controversy is described in the wikipedia article:
http://en.wikipedia.org/wiki/Mole_(unit) 


#3
Dec312, 07:30 AM

P: 260

I have basically the same point as the wikipedia article states. So why is it still called a fundamental unit?
Also for kelvin  why is it a fundamental unit as it represents energy 


#4
Dec312, 07:44 AM

P: 587

Why mole and kelvin are basic units?
As you said, it is just a word that represents a count, much like the word "dozen" or "gross". Do you understand why we need such as number in chemistry? In the chemistry lab you are measuring out quantities in grams, but you need to keep track of how many elementary entities you have, not in absolute terms but relative to other substance that you are also measuring in grams. Have you studied stoichiometry problems yet?
The kelvin temperature scale is unique in that 0 degrees represents the lowest possible temperature, where all thermal motion ceases. 


#5
Dec312, 07:49 AM

P: 260

Well I do understand why we need mole. But why is it a fundamental unit.?
You can just define mole to be 6.022 * 10^23 and use the word. 


#6
Dec312, 07:52 AM

Sci Advisor
PF Gold
P: 2,245

Fundamental units are called "fundamental" because there is no practical way to express them in terms of other units.
The fundamental units in the SI are there because 1) They are usedful and 2) They can be realized, i.e. it is something that can be measured and used to calibrate instruments. It is important to understand that the SI is a practical system of units, ultimately it is is system designed in such a way that we can calibrate instruments in a selfconsistent way. This is why the mole and the Kelvin are there, there is no way to express in in terms of other fundamental units in a way that can be used for calibration/comparissons; and both the mole and the Kelvin are (obviously) very important units so they have to be fundamental. 


#7
Dec312, 08:11 AM

P: 260

Oh I had the wrong idea of what a fundamental unit is. I thought it should represent something physical which we can measure.
Okay now mole and kelvin makes sense. Thank You! 


#8
Dec312, 08:13 AM

Mentor
P: 11,735

http://en.wikipedia.org/wiki/New_SI_definitions#Mole 


#9
Dec312, 08:20 AM

Mentor
P: 15,147

Even in those simple cases (i.e., ideal gases), using energy/mole in lieu of temperature doesn't quite cut it. Consider a vessel that contains two gases separated by an impermeable wall that transmits heat. Put some quantity of an ideal gas in one half, some quantity of another ideal gas in the other half. Heat will be transferred across the wall from one gas to the other if the two gases are at different temperatures. Heat may or may not be transferred if the two gases have different specific energies. For example, one gas is monatomic, the other diatomic. Real gases aren't ideal, making the relationship between temperature and specific energy a nonlinear one. Things get even worse when you consider the fact that gases can condense, liquids can freeze, chemicals can combine. The concept of temperature is very useful and is measurable. Specific energy is less useful, plus how do you measure it? 


#10
Dec312, 10:08 AM

Mentor
P: 11,735

In statistical mechanics, we can define temperature via
$$\frac{1}{T} = {\left( \frac{\partial S}{\partial U} \right)}_{N,V}$$ where ##S = k \ln \Omega## (##\Omega## being the multiplicity of the system). If we wanted to be really fundamental about units, we would make entropy a fundamental quantity, and use the numerical value of k to define the unit of entropy which we might call the "boltzmann" (B). Then the kelvin would be a derived unit: 1 K = 1 J/B. 


#11
Dec312, 10:40 AM

Sci Advisor
PF Gold
P: 2,245

kB will unless I am misstaken actually be defined in 2014. The general "philosophy" of the new SI (which is slowly being introduced) is to have one fundamental constant per unit and then realize the unit by e.g. counting (similar to what we do with the meter and the speed of light).
Hence, the Boltzmann, Avogadros constant, e etc will all eventually be defined to have definite values. 


#12
Dec312, 11:18 AM

P: 2

A mole is just a number like "dozen", it's not a unit of measurement since it doesn't measure anything. A meter is a measurement of length, a second is a measurement of time, a gram is a measurement of mass, and a kelvin is a measurement of temperature, but a mole isn't a measurement of anything.



#13
Dec312, 11:26 AM

Sci Advisor
PF Gold
P: 2,245

Since Avogadro's constant is not a defined number you can't at the moment use that to realise the Mole. Hence, there are other methods for realising the mole, but neither of them directly involves counting anything. 


#14
Dec312, 12:01 PM

P: 887

Basically, Boltzmann's constant is just a historical artifact. It should be regarded as a unit conversion factor between energy and temperature units. In plasma physics, and several other fields, temperature is measured in units of energy (typically electron volts), dispensing with Boltzmann's constant.
D H: Temperature and energy don't have to mean the same thing to use the same units. jtbell: In statistical mechanics, (fundamental) entropy is unitless, being nothing more than the natural logarithm of a number of states. Temperature has the units of energy/entropy, which, therefore, is just the units of energy. The equipartition theorem states that the average energy in an accessible degree of freedom is 1/2 the temperature times the Boltzmann's constant. If we get rid of the Boltzmann's constant, we can just give temperature in energy units, which is a lot more natural and simple. 


#15
Dec312, 12:53 PM

P: 4

Since you can determine the quantity of molecules or atoms of a substance by it's molecular mass/mole, you can figure out how many grams of a substance to add to another substance to predict a chemical reaction.(because molecules combine in predictable quantities with other molecules) The 'gram' measurement is related to the mass of one cubic centimetre of H20. A meter is the length of 100 cm, and is otherwise recorded somewhere as the number of wavelengths of a certain frequency of light, or the distance light travels in a certain time. I think that length is a truly arbitrary value (check an historical reference), and water is an arbitrarity chosen element (for historically obvious reasons). 


#16
Dec312, 01:25 PM

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#17
Dec312, 03:04 PM

P: 928

http://en.wikipedia.org/wiki/Mole_(unit) 


#18
Dec312, 03:23 PM

P: 587




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