### Formula Units

Posted:

**Wed Apr 11, 2018 10:32 am**In practice problem E25, it asks about the number of formula units in 0.750 mol KNO3. What are formula units?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=8&t=30595

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Posted: **Wed Apr 11, 2018 10:32 am**

In practice problem E25, it asks about the number of formula units in 0.750 mol KNO3. What are formula units?

Posted: **Wed Apr 11, 2018 10:34 am**

Formula units is the units used for when converting mass or moles using Avogardro's number. Formula units are used in this case because KNO3 is not a covalent compound.

Posted: **Wed Apr 11, 2018 10:41 am**

So in e21, it says to calculate moles and the number of molecules and formula units/atoms in a) 10.0 g of alumina Al2O3. I know how to convert that to moles, since you just divide by the molar mass of alumina, and for the number of molecules, you multiply the number of moles by Avogadro's number. But how would you convert that to formula units?

Posted: **Wed Apr 11, 2018 10:43 am**

In this case, alumina is a covalent bond. Covalent bonds use the unit molecules when converting using Avogadro's number.

Formula units are used when converting an ionic bond.

Formula units are used when converting an ionic bond.

Posted: **Wed Apr 11, 2018 10:46 am**

So for covalent bonds, you would answer with something like 6.022*10^23 molecules, and then for ionic bonds, you would answer with 6.022*10^23 formula units?

Posted: **Wed Apr 11, 2018 10:46 am**

Yes.

Also just to add, if you are converting a single element using Avogadro's number, you would use atoms as the unit.

Also just to add, if you are converting a single element using Avogadro's number, you would use atoms as the unit.