Answer:
a. Lead (Pb)
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Answer:

Explanation:
Hello!
In this case, since the given reaction is:

Whereas the equilibrium constant is:
![K=\frac{[C][D]^{5/2}}{[A][B]^2} =4.0](https://tex.z-dn.net/?f=K%3D%5Cfrac%7B%5BC%5D%5BD%5D%5E%7B5%2F2%7D%7D%7B%5BA%5D%5BB%5D%5E2%7D%20%3D4.0)
However, the new target reaction reverses and doubles the initial reaction to obtain:

Whereas the equilibrium constant is:
![K'=\frac{[A]^2[B]^4}{[C]^2[D]^5}](https://tex.z-dn.net/?f=K%27%3D%5Cfrac%7B%5BA%5D%5E2%5BB%5D%5E4%7D%7B%5BC%5D%5E2%5BD%5D%5E5%7D)
Which suggest the following relationship between the equilibrium constants:

So we plug in to obtain:

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There are four Hydrogen atoms in one molecule of Methane (CH₄).
And there are 6.022×10²³ molecules in 1 mole of CH₄.
So,
Number of Hydrogen atoms is 1 mole of CH₄ are,
= 6.022 × 10²³ × 4
= 2.4 ×10²⁴ Hydrogen Atoms
Now calculating for 2 moles,
As,
1 mole of CH₄ contains = 2.4 ×10²⁴ Hydrogen Atom
Then,
2 moles of CH₄ will contain = X Hydrogen Atoms
Solving for X,
X = (2 moles × 2.4 ×10²⁴ Hydrogen Atom) ÷ 1 mole
X = 4.8 × 10²⁴ Hydrogen Atoms