Question

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which contains more molecules 4 gram of Methane or 4 gram of oxygen

Answer 1

We know that greater the number of molecules of a substance, the more moles of a substance present

In case of methane(CH4)

Number of moles= Given mass/ Molar mass

Number of moles of methane= 4g/ 16g

= 0.25g

So for methane: 0.004/(12+(1*4))=2.5x10^-4 mols (mass is in kilograms)  

and for O2: 0.004/(2*16)=1.25x10^-4  

So methane will have more number of molecules.

Answer 2

You can consider the number of moles and the molar mass of a compound in order to get the appropriate answer.

They both must be equal.

Here methane will have 4g/16g = 0.25 g of moles in total.

Hence, total molecules will be number of moles * Avogadro number

= 1.5 * 10^23

Similarly, in the case of oxygen, the total moles will be 4g/8g/ = 0.125g

Hence, the molecules will be 0.75 * 10^23

Hence, methane has more molecules.