**Answer: We have**

1 amu = 1.66 x 10^{-24} grams

126 amu = 126 x 1.66 x 10^{-24}

= 209.16 x 10^{-24} grams

1 mole HNO_{3} = 63.01 g

Hence 209.16 x 10^{-24} grams = 3.32 x 10^{-24} moles

1 mole = 6.023 x 10^{23} molecules

hence 3.32 x 10^{-24} moles = 19.99 x 10^{23 }molecules

1 molecule of HNO_{3 }contains 5 atoms.

Hence, 19.99 x 10^{23 }molecules = 99.96 x 10^{23} atoms

**Therefore, 126 amu of HNO3 contains 99.96 x 10**^{23} atoms.