Answer: 17.2 g 
Step-by-step explanation:
To calculate the moles :

 

The balanced chemical equation is:
 
According to stoichiometry : 
4 moles of 
 produce = 4 moles of 

Thus 0.085 moles of 
 will require=
 of 

Mass of 

Thus 17.2 g of 
 will be produced form 20.0 g of HgS.