E21 (molecules vs formula units)

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Mackenzie Stockton 2H
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Joined: Wed Sep 30, 2020 10:11 pm

E21 (molecules vs formula units)

Postby Mackenzie Stockton 2H » Wed Oct 07, 2020 10:06 pm

E.21 Calculate the amount (in moles) and the number of molecules and formula units (or atoms, if indicated) in (a) 10.0 g of alumina, Al 2 O 3 ; (b) 25.92 mg of hydrogen fluoride, HF HF; (c) 1.55 mg of hydrogen peroxide, H 2 O 2 ; (d) 1.25 kg of glucose, C 6 H 12 O 6 ; (e) 4.37 g of nitrogen as N N atoms and as N 2 molecules.

I know how to go about solving this problem in terms of converting grams to moles and moles to "objects" via avogadros number, but I was wondering if someone could clarify the difference between a molecule and a formula unit in the context of this problem? I know, for hydrogen peroxide, the molecule is H2O2. Would the formula units be H2 + O2, or is the formula unit the lowest ratio of atoms like the empirical formula, or something else? The definition of a formula unit in the textbook is confusing me

Vivian Hoang 1H
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Joined: Wed Sep 30, 2020 10:09 pm

Re: E21 (molecules vs formula units)

Postby Vivian Hoang 1H » Wed Oct 07, 2020 10:14 pm

In terms of this problem, you do use all the atoms except for part A; like you said, H2 + O2. A is asking for just alumina so it does not refer to the O3.

For example, you find the moles of HF by turning 25.92 mg into grams and then divide by the molar mass of HF. After that, multiply by Avogadro's number to get the formula units.

Isaac Wen
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Re: E21 (molecules vs formula units)

Postby Isaac Wen » Thu Oct 08, 2020 12:05 am


I think I've heard somewhere that formula units usually refer to ionic compounds while molecules refer to elements that are covalently bonded, if that helps.

Tessa House 3A
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Re: E21 (molecules vs formula units)

Postby Tessa House 3A » Thu Oct 08, 2020 12:31 am

I think that generally molecules refer to covalently bonded atoms and formula units refer to atoms bonded with an ionic bond. This is because an ionic bond does not form a molecule. Therefore, it is called a formula unit. This problem is asking for the moles to be calculated, as well as molecules or formula units.

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