# Amount Of Substance as A Unit

1. Jul 16, 2009

I never really understand why we should have a 'unit' for a 'count' i.e. the mole. And what is the need of assigning a separate quantity, the amount of substance... we already have 'mass'; don't we ?

P.S. - I mistyped the topic title... It should be "Amount Of Substance as a quantity"

Last edited: Jul 16, 2009
2. Jul 16, 2009

### Nick89

And how do you think mass is a measure of the 'amount of substance'? If you have one kilogram worth of feathers, and one kilogram worth of steel, do you feel the 'amount of substance' is the same?

3. Jul 16, 2009

### Staff: Mentor

The "mole" is only needed if there is some indivisible minimum mass of a substance....

4. Jul 16, 2009

### flatmaster

You need to define a counting number as a unit so that you can convert from a discrete scale "number of molecules" to what appears to be a continuous number "number of kilograms". It completes the step from number, to amount.

5. Jul 17, 2009

Err... that depends on how we define 'amount of substance'

So we have a number (one mole) as a unit ? Appears strange...

6. Jul 17, 2009

### Staff: Mentor

Why does it appear strange? You aren't explaining why you are uncomfortable with the concept.

You do understand what atoms and molecules are, right?