How do we get moles from mass?

Answer 1

One mole of a molecule or element is related to the molecular weight of the molecule. Thus the molecular weight of water H2O is 1+1+16 (the atomic weights of the atoms in one molecule of water) is 18 grammes.

The actual figure is slightly different since for more accuracy all atoms have slightly under or slightly over what you'd expect.

This is because of the presence of isotopes and the fact that all atomic weights are based around carbon 12, being exactly 12.00000. But on the Periodic Table carbon appears as 12.0107 because of the presence of it's isotope carbon 14 (2 more neutrons in it's nucleus).

Thus Hydrogen's atomic weight is 1.00794 - but this an average because of the isotopes of hydrogen present - deuterium and tritium. Pure Hydrogen 1, is 1.007825 slightly over 1 because of carbon 12 being the standard.

Oxygen is 15.9994, slightly below 16 again because of small amounts of 2 other isotopes and the carbon 12.0000 contribution.

So the molecular water is actually 18.01528, and so one mole weighs 18.01528g.

One mole of any element or molecule contains the Avogadro's number of atoms/molecules. Avogadro's number is 6.02214179 ×10 to the power of 23, a very large number indeed. The actual weight of a single atom or molecule can be calculated by dividing it's exact molecular/atomic weight by Avogadro's number. Thus one atom of hydrogen 1 weighs 1.6735245 x 10 to the power of minus 22 grammes.

Incidentally, mono molecular water is water vapour (not steam, but the invisible bit you see coming out of a kettle spout at the boil, steam is water droplets). Water as we know it is 3 molecules bound together through hydrogen bonding into a 3 dimensional shape. Water has about 11 or more known forms. This why water is a liquid whereas the much heavier hydrogen sulphide is a gas (no hydrogen bonding)

One mole of any gas will occupy 22.710 980 litres at 0°C, 24.789 598 at 25°C. So just 18g of water in it's monomolecular gaseous state will occupy approx 22.7 litres about 40 pints or 5 UK gallons. Alternatively one pint of water 568 g, is 31.55 moles occupying 716.3 litres or 1260 pints. 157.6 UK gallons as a gas.

Answer 2

it is a simple stoichiometry question. you use the mass given and divide by the molar mass of the substance. the if not given mass in grams convert by using a simple unit multiplier. the final answer will be the ending number with the unit as moles.

Answer 3

moles is a measure of a element or compound when its 6.2something million atoms while molecules is a statement referring to a very small amount of matter

Answer 4

Calculate:

mass/molar mass = moles

Answer 5

We need molarmass for it. divide mass fromthe molar mass.

Answer 6

you just divide by the molar mass