How to solve any stoichiometry problem?

Which of the following represents the ester functional group? a. -coo- b. -cho c. -cooh d. c=o

Asample of hydrated tin (ii) chloride (sncl2) has a mass of 4.90 g. when it is dehydrated, it has a mass of 4.10 g. which is the correct chemical formula for the hydrate? sncl2•2h2o          sncl2•4h2o          sncl2•6h2o

Reactions in cells take place at about a. 40°c b. 0° c. 100°c d. 60°c