How can we determine the number of molecules in a substance accurately? 

The numbers given above are not the real mass of the atoms. What may be the method of stating the mass of atoms? What do you understand from the statement that the atomic mass of Helium is $4$? 

See the chemical equation given for the reaction of burning of carbon in oxygen to form carbon dioxide. 

$\mathrm{C}+{\mathrm{O}}_2\to {\mathrm{CO}}_2$

How many oxygen atoms combine with one carbon atom?

See the chemical equation given for the reaction of burning carbon in oxygen to form carbon dioxide. 

$\mathrm{C}+{\mathrm{O}}_2\to {\mathrm{CO}}_2$

How many oxygen atoms combine with $1000$ carbon atoms?

How many GAMs is present the $42 \mathrm{g}$ of nitrogen? Calculate the number of atoms present in the sample? (Atomic mass of $\mathrm{N}=14$)