To work out the number of moles in an element/compound, just divide the mass by the Ar or the Mr.
“How many moles are there in 32g of oxygen?”
Ar of oxygen is 16
So 32/16 = number of moles
Number of moles = 2
To work out the mass, multiply the number of moles by the Ar or Mr.
“What is the mass of 6 moles of Carbon?”
Ar of Carbon is 12
So 12 x 6 = Mass
To work out the number of particles, just multiply the number of moles by
Avagadro's constant For example:
To work out the number of particles, just multiply the number of moles by Avagadro's constant
“How many particles are there in 2 moles of carbon?”
2 x 6.02 x1023
= 1.2 x 1024
To work out the empirical formulae of a substance, the mass is needed.
“0.020g of Hydrogen combine with 0.168g of Oxygen. What is the empirical formula of Water?”
Mass: 0.020g H 0.168g O
Number of Moles: 0.20/1 0.168/16
Ratio: 0.02 : 0.01 = 2:1 so the formula is H2O
To work out the molecular formula, divide the Molecular Mass by the sum of the Atomic masses of the empirical formula.
“The empirical formula of methane is CH3, the formula mass of methane is 30, work out the molecular formula”
CH3 = 12 + (3x1)
so the molecular formula is twice the empirical formula.