Textbook Problem E.25

[Christine Nguyen 3D]

The question is:

(a) Determine the number of KNO3 formula units in 0.750 mol KNO3. (b) What is the mass (in milligrams) of 2.39 x 10^20 formula units of Ag2SO4? (c) Estimate the number of NaHCO2 formula units in 3.429 g of NaHCO2, sodium formate, which is used in dyeing and printing fabrics.

Does anyone know what exactly are "formula units"? I've never seen the term before and was wondering if I could get help in starting to solve all parts to this problem.

Thanks!

[tamara masri_3D]

Hello! Formula units are quite literally the number of "units" you have of a molecule with a specific formula so one molecule of KNO3 would be one formula unit. As for solving the problem, you can do that using the molar masses of the compounds. Hope this helps!

[Hailey Qasawadish 2J]

Hi! To answer your first question, formula units are simply just the "units" for an iconic company. Similarly, the formula weight would be the molar mass of the ionic compound.

To solve for #25a) you would multiply the moles of KNO3 by Avogadro's number to get how many formula units are in 0.750 moles of KNO3. (Answer: 4.52 x 10^23 formula units)
25b) Divide the given formula units by Avogadro's number and then multiply by the molar mass of Ag2SO4. Multiply by 100 mg/1 gram to get answer in mg. (Answer: 124 mg Ag2SO4)
25c) Divide given grams of NaHCO2 by molar mass and then multiply by Avogadro's number to get the number of formula units of NaHCO2. (Answer: 3.036 x 10^22 formula units)

Hope this helps!

[Charlotte Adams 1A]

Hi! A KNO3 molecule would be the formula unit in this problem. I googled the definition to make sure I was correct and I actually found an old post from Dr. Lavelle from 2018 in which he states:

"formula units mean units of that formula.

For example, suppose one has a small NaCl salt crystal is made up of 9 NaCl compound. Then this crystal has 9 NaCl formula units in it."

Hope this helps!