The mole concept
•Carbon-12 isotope has a relative mass of 12. To calculate the number of carbon-12 atoms that are present in 12g of carbon-12 we need to know the mass of 1 carbon-12 atom. •1 atom of carbon-12 = 2.004 x 10-23 g •X atoms of carbon-12 = 12 g •X = 12 / 2.004 x 10-23 = 6 x 1023 atoms of carbon-12 •The word mole (symbol, mol) represents a number. •A mole is defined as the amount of substance that contains as many elementary particles (atoms, molecules or ions) as there are atoms in 12 g of carbon-12 isotope. •The number of atoms or ions in 1 mole of an element or compound is 6.02 x 10^23 and is called Avogadro’s number (NA). •The molar mass (M) of an element is defined as the mass of 1 mol of the element expressed in grams per mole (gmol-1). •Molar mass is calculated by adding the relative atomic masses (Ar) of each atom of each element present in the formula. Examples: Carbon: M (C ) = 12 gmol-1 Hydrogen: M (H2) = 2 x 1 = 2 gmol-1 Water: M (H2O) = 2 x 1 + 16 = 18 gmol-1 Copper Sulfate: M (CuSO4) = 63.5 + 32 + 4 x 16 = 159.5 gmol-1 Changing moles to particles: Example: A sample of chlorine gas (Cl2) has 4.0 mol of substance. Calculate the number of chlorine molecules. Number of chlorine molecules = n x 6.02 x 10^23 = 4.0 x 6.02 x 10^23 = 2.4 x 10^24 Calculate the number of chlorine atoms. 1 molecule of chlorine (Cl2) contains 2 atoms of chlorine (Cl). Number of chlorine atoms = 2 x number of chlorine molecules = 2 x 2.4 x 10^24 = 4.8 x 10^24 Changing moles to mass Example: Calculate the amount of substance, moles, of 36 g of water, H20. n = m / M = 36 / (2 x 1 + 16) = 36 / 18 = 2 mol Example: Calculate the mass of 5.0 moles of carbon dioxide, CO2 m = n x M = 5.0 x (12 + 2 x 16) = 5.0 x 44 = 220 g |
Changing mass to particles
•To calculate mass from number of particles requires 2 calculations. •Step 1: Calculate amount of substance (n) from number of particles using the rule. n = number of particles / 6.02 x 10^23 •Step 2: Calculate mass from amount of substance using the rule. m = n x M Example: What is the mass of 3.01 x 10^23 molecules of hydrochloric acid, HCl n (HCl) = 3.01 x 1023 / 6.02 x 10^23 = 0.5 mol m (HCl) = n x M = 0.5 x (1 + 35.5) = 0.5 x 36.5 = 18.25 g |