In stoichiometry, chemists are mainly concerned with:
A. the types of bonds found in compounds.
B. mass relationships in chemical reactions.
C. energy changes occurring in chemical reactions.
D. the speed with which chemical reactions occur.

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.  

each chemical family is represented as a group or column on the periodic table. based on chemical properties, there are 6 types of chemical families. group 1 represents alkali metals, group 2 represents alkaline earth metals, groups 3 to 12 represent transition metals, groups 13 to 16 represent non-metals and other metals, group 17 represents halogens and group 18 represents noble gases.