Thu November 24, 2011 By:

why do we need two formulas to take out the number of moles? first formula is:no. of mass=given no. of particles/avogadro's number 2nd is :no. of moles=given mass/molar mass. why so?

Expert Reply
Thu November 24, 2011

For different data provided, we have different formulae. How so ever, all these formulas are inter-related.  A mole is defined as the quantity of a substance that has the same number of particles as are found in 12.000 grams of carbon-12. This number, Avogadro's number, is 6.022x1023.

1 mole of helium atoms (He) contains 6.022 x 1023 helium atoms (He).


The mass in grams of one mole of a compound is equal to the molecular weight of the compound in atomic mass units.

So, one mole of a compound contains 6.022x1023 molecules of the compound. The mass of 1 mole of a compound is called its molar weight or molar mass. The units for molar weight or molar mass are grams per mole. Here is the formula for determining the number of moles of a sample:

mol = weight of sample (g) / molar weight (g/mol)

Also, to find the moles of atoms, 
    divide the number of atoms by 6.022 x 1023 
    eg, If we have 4.2154 x 1023 neon atoms, how many moles of neon atoms are there?
    Moles of neon atoms = (4.2154 x 1023) ÷ (6.022 x 1023
    = 0.7 mol



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