In 1985 benitoite became the California "state gemstone." Found only in a tiny mine near Coalinga, California, it is a silicate of barium and titanium with trace impurities that cause a range of hues from colorless to blue to pink. Its general formula is BaTi(SiO3)3. If a 15 carat stone were pure BaTi(SiO3)3, how many moles of silicon would it contain? (There are exactly 5 carats per gram)

Answer 1

Total mass = 15x5 = 75g of BaTi(SiO3)3

n = m / M

n = 75g / 413.45 g/mol

n = 0.181 moles BaTi(SiO3)3

0.181 moles BaTi(SiO3)3 * (3 moles Si / 1 mole BaTi(SiO3)3

= 0.544 moles of Si

Answer 2

Total mass = 15x5 = 75g of BaTi(SiO3)3 should be
15 carats x (1 g/5 carats) = 3 grams and go from there.