I am going to be installing ceramic tile in a small bathroom that currently has old 12 x 12 vinyl tiles on the floor. I've pulled most of the old tiles off and I've discovered that the adhesive used for the old tiles is still there. It looks (although I know that it isn't) like roofing cement or some tarlike substance. There is enough of it left in spots that you can actually make an indentation with a fingernail. I would like to know if this adhesive needs to be removed and if so, what would be the best way. Personally, because of how soft it is, I think that scraping may be difficult. The adhesive is stuck to a concrete slab, which will be the base for my new ceramic tile. Also, is it mandatory to add a latex additive to thinset mortar? Thank you very much.