How many moles of ethyl alcohol, c2h6o, are in a 21.0g sample

C₂H₆O :

From Periodic table C = 12, H = 1, O = 16

C₂H₆O = 2*12 + 6*1 + 16*6 = 126 g/mole

126g  contains 1 mole of C₂H₆O

1g would contain  (1/126) mole of C₂H₆O

Therefore 21g would then contain:       21*(1/126) = 21/126 ≈ 0.1667

21g of C₂H₆O contains ≈ 0.1667 moles.

The chemical bubbles

Only one reactant yields 2 or more products. meaning you will only have 1 product and it will break down into 2 or more products. eg. h2o -> h2 + o2