Homework Help: Chemistry

Posted by Jay on Monday, April 18, 2011 at 7:17pm.

Calcium fluoride is the well-known mineral fluorite. It is known that each unit cell contains four Ca2+ ions and eight F- ions and that the Ca2+ are arranged in a face-centered cubic lattice. The F- ions fill all the tetrahedral holes. The edge of the CaF2 unit cell is 5.46295 x 10^-8
cm in length. The density of the solid is 3.1805 g/cm^3. Use this information to calculate Avogadro's number.

I am completely lost on what to do. Here is what I have tried to do so far.

5.46295 x 10^-8 cm (taken to the third power)= 4.63035 x 10^-22 cm^3.

Then I utilized the density to convert to grams.

1.63035 cm^3 x 1g/3.1805 cm^3 = 5.1261 x10^-23 grams

Here is where I get lost. I tried using the molar mass of CaF2, but it's not equal to Avogadro's number.

Any pointers would be greatly appreciated!

Thank you,

Jay

Chemistry - Jay, Monday, April 18, 2011 at 10:43pm
Oh just kidding after much frustration and stepping away from the problem and coming back, I noticed that I messed up the density in my conversion. I had the right idea I just had to correct my mistake and then continue on with the molar mass of CaF2, then I divided 4 molecules by the calculated moles, to get Avogadro's number.

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