Weight percent of Ge needed to yield alloy with x atoms/cm3

[physicsnnewbie]

Homework Statement

Germanium forms a substitutional solid solution with silicon. Compute the weight percent of germanium that must be added to silicon to yield an alloy that contains 2.43e21 Ge atoms per cubic centimeter. The densities of pure Ge and Si are 5.32 and 2.33 g/cm3 respectively.

Homework Equations

not sure

The Attempt at a Solution

calculate Ge weight per atom:
72.6/6.0221418e23 = 1.20555e-22

calculate g/cm3 germanium in alloy
1.20555e-22*2.43e21 = 2.92949e-1

I'm not sure what to do next.

 

[Borek]

How many atoms of germanium per cubic centimeter in pure germanium?

How many atoms of silicon per cubic centimeter in pure silicon?

 

[physicsnnewbie]

Ah, thanks Borek! I calculated the atoms per cc for Si and Ge as you said, subtracted the Ge atoms needed per cc from the Si since the Ge was the lower of the two, calculated the weight of both and used that to determine weight percent.