## Formula units vs molecules vs atoms? [ENDORSED]

Victoria Li 1L
Posts: 51
Joined: Sat Jul 20, 2019 12:15 am

### Formula units vs molecules vs atoms?

What exactly is the difference between formula units, molecules, and atoms? And what is a formula unit in general? How would you calculate the number of formula units given a compound's mass? For reference, this is the HW question I'm referring to (Fundamentals E)

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, Al2O3; (b) 25.92 mg of hydrogen fluoride, HF; (c) 1.55 mg of hydrogen peroxide, H2O2; (d) 1.25 kg of glucose, C6H12O6; (e) 4.37 g of nitrogen as N atoms and as N2 molecules.

Katherine Wu 1H
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Joined: Fri Aug 30, 2019 12:15 am
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### Re: Formula units vs molecules vs atoms?

They're looking for molar mass, n=(amount of moles) and N=(number of objects).

find molar mass of Al2O3.
10g/molar mass = n.

Ruby Tang 2J
Posts: 102
Joined: Fri Aug 30, 2019 12:15 am

### Re: Formula units vs molecules vs atoms?  [ENDORSED]

A formula unit is the empirical formula of any ionic solid (or covalent network solid). Since these solids (ex. NaCl or SiO2) don't exist as individual molecules, we use the term "formula unit" instead. In the example you stated, alumina is an ionic compound so the technically correct unit is "formula unit," whereas for glucose, a non-network solid held together by covalent bonds, the technically correct unit is "molecule". There isn't really a difference in how you calculate for one or the other; the terms are analogous.

ALegala_2I
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Joined: Thu Jul 11, 2019 12:17 am

### Re: Formula units vs molecules vs atoms?

I am still confused on the concept of formula units. How many formula units would be in 46g of NaCl?

Ruby Tang 2J
Posts: 102
Joined: Fri Aug 30, 2019 12:15 am

### Re: Formula units vs molecules vs atoms?

46 g of NaCl * (1 mol of NaCl/58.44 g of NaCl) = 0.787 mol of NaCl (6.022 * 10^23 formula units/1 mol of NaCl) = 4.7 * 10^23 formula units of NaCl. You would essentially solve it the same way as you would for any molecular compound; you are just substituting the word "molecule" for "formula unit" because this is our way of dividing up a sample of an ionic compound into individual entities for the purpose of calculation. Hope this explanation is clearer!

JasonLiu_2J
Posts: 109
Joined: Sat Aug 24, 2019 12:17 am

### Re: Formula units vs molecules vs atoms?

Just to add on to what was previously stated, a formula unit is just another organizational term/unit for a particular object. Avogadro's constant (6.022 x 10^23 "objects" per mole) indicates the number of individual objects in a mole of said object. A single atom is the smallest unit of matter that one can have, and is the organizational unit used when describing the number of "objects" of individual elements. For example, you can have 1 mol of C atoms, which is equivalent to 6.022 x 10^23 individual Carbon atoms. A molecule is a compound of atoms that are covalently bonded together, and is the organizational unit used to describe the number of "objects," which in this case are the individual number of molecules. E.g. you can have 1 mol of H2O molecules, which is equal to 6.02 x 10^23 individual molecules of H2O. In the same way, a formula unit is an organizational unit for molecules that do not appear individually, but rather in ionic solids or network covalent solids (The molecules of these empirical formulas appear together, whereas you want to find the number of individual substances that have that empirical formula, thus the formula unit). Instead you are finding the number of individual "objects" with this empirical formula, so the number of objects in question is the number of formula units. You would solve for the number of formula units in the same way as you would solve for the individual number of atoms or molecules when given a mass of the particular substance. Use the molar mass of the element or formula to convert mass to moles of the substance and then use Avogadro's constant to convert the number of moles to the number of individual objects you want to find, whether it be the individual number of atoms, molecules, or formula units.

Victoria Li 1L
Posts: 51
Joined: Sat Jul 20, 2019 12:15 am

### Re: Formula units vs molecules vs atoms?

Ruby Tang 1E wrote:A formula unit is the empirical formula of any ionic solid (or covalent network solid). Since these solids (ex. NaCl or SiO2) don't exist as individual molecules, we use the term "formula unit" instead. In the example you stated, alumina is an ionic compound so the technically correct unit is "formula unit," whereas for glucose, a non-network solid held together by covalent bonds, the technically correct unit is "molecule". There isn't really a difference in how you calculate for one or the other; the terms are analogous.

That makes sense - thanks so much!

Victoria Li 1L
Posts: 51
Joined: Sat Jul 20, 2019 12:15 am

### Re: Formula units vs molecules vs atoms?

ALegala_3I wrote:I am still confused on the concept of formula units. How many formula units would be in 46g of NaCl?

According to the explanations on this page, I'm assuming it would be the number of NaCl "molecules" in 46g. I put molecules in quotations since NaCl wouldn't be a single molecule in real life, but split into separate Na and Cl ions in solution. So we would probably just convert 46g to moles and then use Avogadro's number to convert it to "molecules." Hope that helps!