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Here we have 126 AMU of HNO3,

Molar mass of HNO3 is 1+14+16×3 = 63amu

Number of molecules of HNO3 are = 126/63 = 2

Here we have 2 molecules

Each molecule have 3 atoms of oxygen.

So no. of oxygen atoms= 3×2 =6 atoms

No. Of moles = given particles/ avogadro number

No. of moles of oxygen = 6 / 6.022 x 10²³  

No. of moles =9.96 x 10²²

for complete explanation of mole concept click the link below:-