# Moles

Avogadro Constant: One mole = 6.02 x1023 particles, easier way to count number of particles. As
mole linked to carbon- 12. 1 mole in grams = relative atomic mass.
• Number of atoms = 6.02 x1023 x molar mass. 0.12 moles of S8 molecules means need to multiply
by 8 as actually working out molar mass, not moles.
• Moles (n) = Mass/ Molar Mass.
• Mass must be in grams. Molar mass in units g mol-1
.
• How many molecules? Use Avogadro’s constant.
• How many bromine molecules present in each dm3 when bromine air is at a concentration of 4 x
10-6
gdm-3
? 4 x 10-6
/ 79.9 x 2 = mass/ Mr = 2.5 x10-8 moles. Then multiply by 6.02 x1023 to find
number of molecules.
• Remember bromine air or water is Br2 unless stated otherwise.
• A natural gas supply contains 16.0% H2S by volume. The H2S (g) in 1.50 × 108 dm3 of this natural
gas supply, measured at RTP, is processed into sulfur with an overall percentage yield of 95.0%.
Calculate the mass of sulfur, in g, obtained from 1.50 × 108 dm3 of natural gas supply.
– 0.16 x 1.50 × 108 = volume of H2S. Work out moles.
– Use molar ratio to find moles of S which is theoretical moles.
– Rearrange % yield formula to find actual moles of S. Find mass.