How many moles of hydrogen gas would be needed to react with excess carbon dioxide to produce 42.1 moles of water vapor?

Question

This is a really tough subject for me.

Answer 1

CO₂ + 4H₂ ===> CH₄ + 2H₂O ... balanced equation

Carbon dioxide plus hydrogen gas ==> methane + water

If you have 42.1 moles of H₂O, we can use the balanced equation to find the moles of H₂ needed, since the mole ratio of H₂ to H₂O is 4 : 2 = 2 : 1.

42.1 moles H2O x 4 moles H2/2 moles H2O = 84.2 moles H₂ needed

Answer 2

Start with a balanced equation. The leading coefficients of the equation become your reference.

2AgNO₃(aq) + Na₂CrO₄(aq) → Ag₂CrO₄(s) + 2NaNO₃(aq) says that 2 moles of silver nitrate react with 1 mole of sodium chromate to create 1 mole of silver chromate and 2 moles of sodium nitrate.

Your equation, before it's balanced will be (A)H₂ + (B)CO₂ -> (C)H₂O + (D)C₂.

Once you balance it, the ratio will be A/C = X/42.1.

Then, you solve for X.