[CFG]

Question from the text:

An alloy used in an artificial hip contains 17 g of Ni, 23 g of Cr, and 40 g of O. Calculate the mole fractions and mass fractions of each element in the alloy. Also, calculate the average molecular weight of the alloy.

What I've done:

I've calculated the mass and molar fractions, as requested.

Mass:

Nickel: 17g/80

Cr: 23g/80

O: 40g/80

Moles:

0.29 mol Ni, 0.44 mol Cr, 2.50 mol O

My problem:

What I'm having a problem with is the molecular weight - Since this ends up as 0.29 mol Ni, 0.44 mol Cr, and 2.5 mol O, these don't seem to bond into any molecule I can find. Since I can't find any molecule with anything close to these ratios, I can't find the molecular weight. The only other thing I can think of is that the Cr and Ni are alloyed into Nichrome, but then there's all this oxygen left floating around. Do y'all have any advice?

[Borek]

Mixture is not a compound, so no wonder you can't find any molecular formula. My bet is that you are expected to calculate weighed average "as if" the composition was Ni_xCr_yO_z (where x,y,z are molar fractions).

[CFG]

Thanks - That was my other thought, but I'm still confused as to how oxygen is supposed to remain in the alloy.

[mjc123]

You haven't calculated the mole fractions. Mole fractions add up to 1.
(Strictly, you haven't calculated the mass fractions either, only given an expression which you haven't evaluated.)
The "average molecular weight" is the mass of 1 mole of substance of molar composition Ni_xCr_yO_z, where x + y + z = 1.

[CFG]

Thanks, I did calculate those on paper and forgot to add them to the post. I ended up calculating that average molecular weight, but I'm still confused as to the practicality of having that much oxygen in an alloy

[Borek]

I'm still confused as to the practicality of having that much oxygen in an alloy

You are not alone, can be some error in the question wording. Still, in this case you have no choice but to deal with what you are given.

[Enthalpy]

Co would be more credible than O. Cobalt-based alloys exist with extreme corrosion resistance and are used for prostheses. Cr gives the passivating layer and Ni I don't know, maybe it stabilizes the austenite.
One  example is:
40%Co, 20%Cr, 16%Ni, 7%Mo, 2%Mn
http://www.matthey.ch/fileadmin/user_upload/downloads/fichetechnique/EN/Phynox_C.pdf
https://www.fwmetals.com/services/resource-library/fwm-1058/
https://www.alloywire.com/products/phynox/