Question

How many moles contain 1.25x 10^15 molecules carbon dioxide?

Answer 1

Avagadros number states that 1 mol of any substance is made of 6.022 x 10²³ units.
These units could be atoms that make up an element or molecules that make up a compound. CO₂ is a compound therefore its made of CO₂ molecules.
1 mol of CO₂ contains 6.022 x 10²³ molecules of CO₂
Therefore if  6.022 x 10²³ molecules of CO₂ make up 1 mol 
then 1.25 x 10¹⁵ molecules of CO₂ make up - $\frac{1.25 x 10¹⁵}{6.022 x 10²³}$
Number of CO₂ moles present - 2.0756 x 10⁻⁹ mol of CO₂