E21 (molecules vs formula units)

Mackenzie Stockton 2H
Posts: 102
Joined: Wed Sep 30, 2020 10:11 pm

E21 (molecules vs formula units)

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
Posts: 107
Joined: Wed Sep 30, 2020 10:09 pm

Re: E21 (molecules vs formula units)

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
Posts: 58
Joined: Wed Sep 30, 2020 9:35 pm
Been upvoted: 1 time

Re: E21 (molecules vs formula units)

Hi!

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
Posts: 101
Joined: Wed Sep 30, 2020 9:52 pm
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Re: E21 (molecules vs formula units)

Hi,
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.