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Formula Unit

Posted: Sat Sep 28, 2019 4:10 pm
by Claire Lo 3C
Could someone please explain the difference between a formula unit and a molecule? Thanks.

Re: Formula Unit

Posted: Sat Sep 28, 2019 4:26 pm
by BritneyP- 2c
So I was reading through some online posts, but I think that formula units are only applied to salts such as NaCl. Molecular formulas are typically covalently bonded and so because salts like NaCl are ionic bonds they aren't molecules. I don't know if that helps but that is just what I think

Re: Formula Unit

Posted: Sat Sep 28, 2019 4:27 pm
by BritneyP- 2c
So I was reading through some online posts, but I think that formula units are only applied to salts such as NaCl. Molecular formulas are typically covalently bonded and so because salts like NaCl are ionic bonds they aren't molecules. I don't know if that helps but that is just what I think

Re: Formula Unit

Posted: Sat Sep 28, 2019 4:32 pm
by Alan Wu
Formula units are typically used to describe one unit of an ionic compound. Ionic compounds are often found in huge lattice structures, where the positive and negative ions alternate. The formula unit would describe the most basic unit that makes up the ionic compound (e.g. NaCl). A molecule is used to describe any compound covalently bonded together (e.g. CO2, H2, etc).

Re: Formula Unit

Posted: Sun Sep 29, 2019 12:25 am
by Matthew Chan 1B
I think a molecular formula gives a list of the sets of atoms that are bonded together. For example, H2O, which is a molecular formula, tells us that there are two hydrogens and one oxygen. The molecular formula gives you the atoms that make up the molecule. A formula unit is more of a ratio of atoms/combined elements in an ionic compound. It tells us how many atoms per other atom. We can see this in MgCl2, where the only thing that we know from reading this formula unit is the ratio of atoms. For every one Mg, there are two Cl. In a formula unit, we only know about the ratio. Hopefully this makes sense! :)

Re: Formula Unit

Posted: Sun Sep 29, 2019 10:39 am
by Ashley Tran 2I
Something that also helps is that a formula unit is defined as the empirical formula of any ionic or covalent network solid compound used as an independent entity for stoichiometry. This relates to what was being said earlier about the formula unit representing ratios. And therefore, there are 6.022 x 10^23 formula units per mole.

Re: Formula Unit

Posted: Sun Sep 29, 2019 11:27 am
by Julia Mazzucato 4D
Are formula units used to describe ionic compound because they easily disassociate in water/aqueous solutions?

Re: Formula Unit

Posted: Sun Sep 29, 2019 5:17 pm
by Matthew Chan 1B
Julia Mazzucato 4D wrote:Are formula units used to describe ionic compound because they easily disassociate in water/aqueous solutions?


I don't think that formula units are used to describe ionic compounds just because they easily disassociate in water/aqueous solutions; Ionic compounds don't exist in nature as individual molecules, but as mentioned by others, they need to be in a crystal lattice structure. There is no such thing as a molecule of an ionic compound, but rather, there are multiple Na+ and Cl- ions in a space. I believe that there is other reasoning why formula units are used to describe ionic compounds other than because they happen to dissociate in water/aqueous solutions.