1. Write the balanced chemical equation:

The reaction between carbon disulfide (CS₂) and hydrogen gas (H₂) produces methane (CH₄) and hydrogen sulfide (H₂S):

CS₂ + 4H₂ → CH₄ + 2H₂S

2. Determine the mole ratio between H₂ and CH₄:

From the balanced equation, we see that 4 moles of H₂ react to produce 1 mole of CH₄.

3. Apply Avogadro's Law:

At STP (Standard Temperature and Pressure), equal volumes of gases contain equal numbers of moles. Therefore, the mole ratio is also the volume ratio. This means 4 liters of H₂ are needed to produce 1 liter of CH₄.

4. Calculate the volume of H₂ needed:

We need to produce 2.5 liters of CH₄. Using the volume ratio from step 3:

(4 L H₂) / (1 L CH₄) = (x L H₂) / (2.5 L CH₄)

Solving for x:

x = (4 L H₂) * (2.5 L CH₄) / (1 L CH₄) = 10 L H₂

Therefore, you need 10 liters of hydrogen gas to react with CS₂ to produce 2.5 liters of CH₄ at STP.