Answer is 1.2 x 1024 atoms. Explanation: Divide 9.03 x 1023 by 6.02 x 1023 to find the number of moles of hydrogen gas. 9.02 / 6.02 = 1.5 moles (remember the numerator and denominator each contain 10 to the 23, so this cancels out. 1 mole equals 6.02 x 1023 molecules).
Let's start from the definition of the Avogadro's number. The Avogadro's number is 6,02 x and means that, for any compound, we have 6,02 x molecules for each mol. So, in that way, we can write the following expression: