The first step to solve this problem is to find how many molecules of scheelite contain 1 billion atoms. To do it, divide 1 billion by 4, this is because each molecule of scheelite contains 4 atoms:
[tex]1.00\times10^9atoms\cdot\frac{1molecules}{4atoms}=2.50\times10^8molecules[/tex]Now, use Avogadro's number to find the amount of moles in 2.50x10^8 molecules:
[tex]2.50\times10^8molecules\cdot\frac{1mol}{6.022\times10^{23}molecules}=4.15\times10^{-16}mol[/tex]Use the molecular weight of scheelite to convert the obtained moles to grams:
[tex]4.15\times10^{-16}mol\cdot\frac{287.84g}{mol}=1.19\times10^{-13}g[/tex]The answer is 1.19x10^-13 grams.