Avagadros constant states that there are 6.022 x 10²³ units in 1 mol of substance.
These units could be atoms that make up an element, or molecules making up a compound or ions as well.
1 mol of O₂ contains - 6.022 x 10²³ molecules of O₂
molecular mass of O₂ is (16*2) = 32 g/mol
1 mol of O₂ weighs = 32 g
Therefore in 32 g of O₂ - 6.022 x 10²³ O₂ molecules
gas tank contains O₂ weighing 650 g
in 1 g - 6.022 x 10²³/32 molecules
in 650 g - 6.022 x 10²³/32 * 650
= 2.96 x 10²² molecules of O₂
