11. _al + _h2s → _al2s3 + _h2

Explanation:

Step1: Balance sulfur atoms

There are 3 sulfur atoms in $Al_2S_3$. So, we need 3 moles of $H_2S$ to balance the sulfur atoms. The equation becomes $Al+3H_2S\rightarrow Al_2S_3 + H_2$.

Step2: Balance aluminum atoms

There are 2 aluminum atoms in $Al_2S_3$. So, we need 2 moles of $Al$ to balance the aluminum atoms. The equation is now $2Al + 3H_2S\rightarrow Al_2S_3+H_2$.

Step3: Balance hydrogen atoms

On the left - hand side, we have 6 hydrogen atoms from $3H_2S$. So, we need 3 moles of $H_2$ on the right - hand side to balance the hydrogen atoms.

Answer:

$2Al+3H_2S\rightarrow Al_2S_3 + 3H_2$