1 mole of any compound has a mass equal to the relative molecular mass of that compound expressed in grams. Don't forget that a mole is just another name for an Avogadro number of particles of that substance - 6,02 x 1023.
Example: water H2O
Relative molecular mass = (2 x 1) + 16 = 18
Mass of 1 mole of water = 18g
18g of water contains 6,02 x 1023 molecules of water.
Each molecule of water has two hydrogen atoms and one oxygen atom .. so 1 mole of water is made up of 2 moles of hydrogen atoms and 1 mole of oxygen atoms*
½ mole of water has a mass of 18 x ½ = 9g **
½ mole of water contains (6,02 x 1023 ) x ½ molecules.
*(of course, the atoms of oxygen and hydrogen are not separate within water but we can still talk about them just the same - it's like saying that twenty people have forty arms and forty legs - the arms and legs are not separate entities but we can still talk about them as if they are)
** ½ can also be written as decimal 0,5. This means that the method for calculation of the mass of a certain number of moles is = the mole fraction x the relative molecular mass.
Homework: Calculate the mass of the following:
Quote - "If you want to know something, look it up. If you want to understand something, figure it out" - Richard Feynmann