# Fundamental problem regarding mole concept

#### donaldparida

Background: Amount of substance is a fundamental physical quantity which has mole (mol) as it's SI unit. Therefore all expressions for amount of substance should have the unit mole on simplification.

Therefore, if A = Amount of substance (normally in moles), m = Mass of the substance in a particular unit (normally in grams), M is the mass per unit amount of substance (normally in grams/mole), then,

A = m/M

If the substance is a molecule then M is known as molar mass of the substance.

But in my textbook and in many websites on the internet, I have encountered the phrase number of moles. I think it refers to the amount of substance or may be the numerical part in the amount of substance. The formulas given there were strange.

If n is the number of moles, m is the mass of the substance in grams and x is the atomic weight (for atoms of elements) or molecular weight (for molecules of elements and compounds), then,

n = m/x

This equation is not dimensionally correct if I am right.

My questions:

First of all i would like to ask whether amount of substance and number of moles refer to the same thing or is it that amount of substance has a unit along with a numerical value whereas number of moles does not have a unit and represents the numerical value in the magnitude of the amount of substance.

Secondly, between equations 1 and 2, which one is completely correct (both in meaning and dimension)?

Thirdly, since in some place I have encountered gram atomic/molecular mass in place of molar mass, i would like to know what are the differences between both and do they have the same units or different units?

#### Buffu

Secondly, between equations 1 and 2, which one is completely correct (both in meaning and dimension)?
The only difference is that in first one you have $g/\mathrm{mol}$ as the unit of denominator whereas it is $g$ in second one. First one is dimensionally correct but it does not matter.

First of all i would like to ask whether amount of substance and number of moles refer to the same thing or is it that amount of substance has a unit along with a numerical value whereas number of moles does not have a unit and represents the numerical value in the magnitude of the amount of substance.
Amount of substance can be in dozens but number of moles will be in moles.

#### Ygggdrasil

Gold Member
mol is just one unit for the amount of a substance and you could use other units as well (a mol of atoms, a dozen eggs, a ream of paper). Amount of substance can be converted to number of moles through Avogadro's constant: 6.02e23/mol.

For example, for one helium atom * mol/6.02e23 = 1.66e-24 mol. Similarly, one helium atom * dozen/12 = 0.083 dozen.

Atomic/molecular weights are generally given in g/mol so taking a mass and dividing it by the molecular weight will give number of moles.

#### donaldparida

@Buffu , so number of moles has the unit moles. Right?

#### symbolipoint

Homework Helper
Gold Member
MOLES is used as a unit of measure, and can be abbreviated "mol". This unit is the count of 6.022*10^23 of whatever pieces are being counted.

DOZEN is used as a unit of measure, abbreviated in whatever way will be useful or understood. This unit is a count of 12 of whatever pieces are being counted.

#### DrDu

@Buffu , so number of moles has the unit moles. Right?
Yes, therefore, I think that amount of substance should be the preferred word, which is also the view upheld by IUPAC.

• Buffu