# Homework Help: Weight percent of Ge needed to yield alloy with x atoms/cm3

1. Dec 2, 2008

### physicsnnewbie

1. The problem statement, all variables and given/known data
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.

2. Relevant equations
not sure

3. 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.

2. Dec 2, 2008

### Staff: Mentor

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

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

3. Dec 2, 2008

### 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.