Use the molar mass of barium carbonate to determine the number of moles in 8 grams. Then we can multiply the moles by #6.022xx"10"^23#, the number of items in a mole, including molecules.
Determine moles #"BaCO"_3"# by dividing its given mass by its molar mass #("197.34 g/mol")#. Since molar mass is a fraction #("g"/"mol")#, we can divide by multiply the given mass by the reciprocal of the molar mass #("mol"/"g")#.
Mole #"BaCO"_3"#
#8"g BaCO"_3xx(1"mol BaCO"_3)/(197.34"g BaCO"_3)="0.04 mol BaCO"_3"#
Molecules #"BaCO"_3"#
To determine molecules of #"BaCO"_3"#, multiply the moles by #6.022xx"10"^23"molecules/mol"#.
#0.04"mol BaCO"_3xx(6.022xx10^23"molecules BaCO"_3)/(1"mol BaCO"_3)=2xx10^22 "molecules BaCO"_3"#
(rounded to one significant figure)
