The Mole and Molar Mass of Compounds

The Mole and Molar Mass of Compounds Internet Activity

Objective: to calculate the molar mass of ionic and molecular compounds.

The meaning of a chemical formula can be expressed on two levels; the molecular or formula unit level and the molar level. The stoichiometric equivalence of two elements in a formula unit is the ratio (by atoms or moles) in which they occur in the formula. For example, in the formula P4O10, the ratio of P atoms to O atoms is 4:10. In one mole of P4O10, there are 4 moles of P atoms and 10 moles of 0 atoms. The molecular, or formula mass of a substance is the sum of all the atomic masses which compose one molecule or formula unit and is expressed in amu's. The gram molecular mass or molar mass of a compound is the mass of a mole of molecules or formula units expressed in grams. The molar mass is the same number as the molecular mass, just a different unit. For example, carbon dioxide (CO2) has a formula mass of 44 amu(12 + 16 + 16) because the one carbon atom has a mass of 12amu and the two oxygen atoms have a mass of 16 amu each. Therefore, the formula mass is also 44amu. The molar mass is 44 grams because one mole of carbon dioxide contains one mole of C atoms (12 grams) + 2 moles O atoms (2 x 16 grams = 32grams). 12 g + 32g = 44 grams.

Activity: Determining molar masses of compounds is a fundamental concept in the study of chemistry. Using the same method of calculating molar masses that you learned in class, determine the molar mass of each of the compounds below. Before you begin, review the procedure at one of the following website: http://www.carlton.paschools.pa.sk.ca/chemical/Molemass/default.htm (Click on “Take the quiz” at the bottom of this page and take the quiz.)

http://www.chem.vt.edu/RVGS/ACT/notes/Study_Guide-Moles_Problems.html

Complete the following calculations: Determine the molar mass of each compound: a. Al2O3 b. BaO c. Ca3(PO4)2 d. KMnO4 e. C6H4Cl2

Check your answers by using the calculator at one of the following websites: How well did you do? http://www.carlton.paschools.pa.sk.ca/cgi-win/mmcalc.exe http://www.chem.vt.edu/RVGS/ACT/notes/molar_mass.html

Continue practicing molar mass calculations at the following sites: http://www.chem.vt.edu/RVGS/ACT/notes/molar_mass.html http://dbhs.wvusd.k12.ca.us/Mole/MolarMass.html (scroll to bottom of page) http://dbhs.wvusd.k12.ca.us/Mole/MolecWt.html (scroll to bottom of page)

Use the electronic calculator to check your answers. Test your knowledge of calculating molar mass at the following website: http://www.carlton.paschools.pa.sk.ca/chemical/molemass/moles6.htm