# Is the largest number of molecules in 36 g of water or 54 g of dinitrogen pentaoxide?

So recently my teacher asked us which of the following has the largest number of molecules? He gave us two options:

1. 36g of $\ce{H2O}$
2. 54g of $\ce{N2O5}$

I'm stuck, because both of them are having two moles of each of the respective molecules. And technically, no. of molecules = mole * Avagadro's number.

So shouldn't be 2 moles * Avagadro's number = same number of molecules for both of them?

But yet, the teacher said that "#1 36g of $\ce{H2O}$" is the correct answer.

And I'm still not able to figure out why?

## 1 Answer

I'm stuck, because both of them are having two moles of each of the respective molecules.

Sure?

$M(\ce{H2O}) = 18\,\mathrm{g\,mol^{-1}}$
$M(\ce{N2O5}) = 108\,\mathrm{g\,mol^{-1}}$

• aw! I did a silly multiplication-division mistake! facepalm Thanks, btw :) – Anoneemus May 16 '15 at 6:17
• @shashikant No problem! Think positive: you understood the concept, it was just the math ;-) – Klaus-Dieter Warzecha May 16 '15 at 6:19