Formula Units  [ENDORSED]

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Phillip Winters 2F
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Joined: Fri Sep 29, 2017 7:05 am

Formula Units

Postby Phillip Winters 2F » Mon Oct 02, 2017 9:22 pm

Problem E9 asks how many formula units of the compound are present and I was wondering what is a formula unit? Is it the just asking how many molecules there are?

Sammy Thatipelli 1B
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Re: Formula Units

Postby Sammy Thatipelli 1B » Mon Oct 02, 2017 9:31 pm

A formula unit is the group of ions that match the formula of the smallest unit of an ionic compound.
When the question is asking for the number of formula units, the procedure is to first calculate the number of moles by dividing the mass of the sample given by the molar mass of the compound. Then, the next step is to calculate the formula units by multiplying by Avogadro's number. Essentially, it is the same process in this case as converting a mass of a sample into molecules.

Ayona Sengupta
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Joined: Fri Sep 29, 2017 7:07 am

Re: Formula Units

Postby Ayona Sengupta » Mon Oct 02, 2017 11:54 pm

The simplest way to think about it, I think, is to just remember that formula units just refer to the number of moles x Avagadro's number. To obtain the number of moles, you'd just need to divide mass by molar mass, as was said previously.

Yang Chen 2E
Posts: 36
Joined: Thu Jul 13, 2017 3:00 am

Re: Formula Units

Postby Yang Chen 2E » Tue Oct 03, 2017 9:28 am

I would have to agree with the second post here. Consider the formula unit like a molecule and use the same method as you would for calculating the number of molecules.

Laura Riccardelli
Posts: 49
Joined: Fri Sep 29, 2017 7:04 am

Re: Formula Units

Postby Laura Riccardelli » Tue Oct 03, 2017 5:11 pm

When you write the answer to question 9 do you follow the number answer with "formula units" or do you leave it blank?

Tanaisha Italia 1B
Posts: 55
Joined: Fri Sep 29, 2017 7:04 am

Re: Formula Units  [ENDORSED]

Postby Tanaisha Italia 1B » Tue Oct 03, 2017 5:34 pm

I would follow it with 'formula units', just because an answer should have a measure of units afterward if possible. In other cases, this allows you to ensure that you are giving the right amount ex. 2 mL is different from 2L.

Kai_Chiu 1F
Posts: 19
Joined: Fri Sep 29, 2017 7:03 am

Re: Formula Units

Postby Kai_Chiu 1F » Tue Oct 03, 2017 8:40 pm

I agree with the above post, I just put "formula units" for my answers!

Michelle Lee 2E
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Joined: Thu Jul 27, 2017 3:01 am

Re: Formula Units

Postby Michelle Lee 2E » Wed Oct 04, 2017 12:02 pm

This post helped me understand the term "formula units". However, in Question E.21, when it asks for "the number of molecules and formula units in ...", should I consider formula units and molecules to be the same thing? If not, how should I look at what this question is looking for?

Sydney Briggs 1B
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Joined: Fri Sep 29, 2017 7:03 am

Re: Formula Units

Postby Sydney Briggs 1B » Wed Oct 04, 2017 2:57 pm

Sammy Thatipelli 1D wrote:A formula unit is the group of ions that match the formula of the smallest unit of an ionic compound.
When the question is asking for the number of formula units, the procedure is to first calculate the number of moles by dividing the mass of the sample given by the molar mass of the compound. Then, the next step is to calculate the formula units by multiplying by Avogadro's number. Essentially, it is the same process in this case as converting a mass of a sample into molecules.


While attempting to figure out what exactly formula units meant, this post clarified it the best. When seeing "formula units" I freaked out, but the inclusion of the entire process allowed me to go back and realize the problem was approachable. Sometimes it is best to look at a problem and take time to reconcile on all the processes you know and apply them. I would also add that when solving for the answer, include formula units in your solution (it is always wise to include units to prevent losing points).

Michelle Lee 2E
Posts: 64
Joined: Thu Jul 27, 2017 3:01 am

Re: Formula Units

Postby Michelle Lee 2E » Thu Oct 05, 2017 11:30 pm

I went back and looked into formula units. The way I understand it now is that formula units are like molecules and atoms; it is in the same category that describes what the atomic item at hand is. A formula unit is different in that it deals with ions (cations, anions). Ions don't behave like molecules in which there is one item that consists of bonded elements. Ions, instead, bond together but do not form what is known as a molecule. For example, salt, NaCl, consists of multiple sodium atoms and multiple chloride atoms. These atoms, however, do not form molecules of separated NaCl bonds but instead form a coagulated mesh of Na+ ions and Cl- ions all organized together. Thus, formula units are to describe how many NaCl "bonds" there are in the coagulated mesh that is the atomic structure of salt.


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