Joanna R. answered 19d
Experienced Science and Math Tutor | Bioengineering Degree | Bilingual
There needs to be a source of sulfur on the left side of the equation in order to have sulfur on the right side of the equation (in the H2SO4). One of the keys to getting the correct answer is to be sure you're copying the problem correctly from one place to another; that's tripped me up before and I suspect what happened here.