How many molecules of hydrogen gas are needed in the chemical equation below? \( \mathrm{CO}_{2}+\mathrm{H}_{2} \rightarrow \mathrm{CH}_{4}+\mathrm{H}_{2} \mathrm{O} \)

In the provided chemical equation, to balance the reaction, you need 4 molecules of hydrogen gas (\( \mathrm{H}_2 \)). The balanced equation is: \[ \mathrm{CO}_2 + 4 \mathrm{H}_2 \rightarrow \mathrm{CH}_4 + 2 \mathrm{H}_2\mathrm{O} \] This ensures that there are equal numbers of each type of atom on both sides of the equation. When dealing with chemical reactions, always remember to balance the number of atoms for each element involved. It helps to start with the least common molecules and work your way to the more complex ones. A common mistake is forgetting to balance hydrogens and oxygens last, leading to confusion and incorrect equations.