How many molecules of o2 are contained in a gas tank that contains 650. g of oxygen?

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

All the elements are arranged by melting points on the periodic table. in row they go from metals to gasses. in the row from sodium to argon, sodium is more reactive than the one on its right, magnesium.