Question

How many moles of N2O3 contain 2.55x10^24 oxygen atoms?

Answer 1

Answer : The number of moles of $N_2O_3$ is 1.41 moles.

Explanation :

In $N_2O_3$, there are 2 nitrogen atoms and 3 oxygen atoms.

As we know that, 1 mole of substance contains $6.022\times 10^{23}$ number of atoms.

As, $3\times 6.022\times 10^{23}$ number of oxygen atoms contains 1 mole of $N_2O_3$

So, $2.55\times 10^{24}$ number of oxygen atoms contains $\frac{2.55\times 10^{24}}{3\times 6.022\times 10^{23}}=1.41$ mole of $N_2O_3$

Therefore, the number of moles of $N_2O_3$ is 1.41 moles.

Answer 2

Moles of O = atom / avogadro's number = 2.55*10^24 / 6.022*10^23 = 4.23 mol 

since each N2O3 contains 3 O atoms: 
moles of N2O3 = moles of O / 3 = 1.41 mol

I searched on google sorry!