Friday, January 8, 2021

Walkthrough: Calculating Molar Mass

 General Chemistry Index

Where are we going with this? This page will assist in developing the ability to perform calculations to determine the composition of a compound or mixture when given the necessary information and to apply lab data to determine the empirical and molecular formula of a compound.

Walkthrough: Calculating Molar Mass
This rings a bell… But… a second look is always a good idea, right?

Calculating molar mass s a part of numerous chemistry concepts; anytime you are dealing with moles and grams, such as in doing stoicheomitry, finding percent composition, and calculating molarity. Let's take a look!

What exactly is the molar mass? Molar mass is the mass in grams of one mole of the substance.

Molar mass us very useful in things like…

This reaction needs 2 moles of iron. How much does two moles of iron weigh?

What is the molarity of a 2 liter solution that contains 284 grams of K2SO4?

The process is pretty simple, actually. Let's check it out!

The setup here is… something something something, you need to know how much one mole of ____ weighs in grams (see useful things above).



What is the molar mass of MgCl2So, we need to find out how much one mole of that compound weighs in grams…

Step 1: Use the molecular formula to identify ALL the TYPES of atoms in the compound.

Let's see… we got Mg and Cl…

Step 2: Use a periodic table to find the atomic masses of each of the elements in the compound.

Mg = 24.3 g/mole

Cl = 35.5 g/mole

Step 3: Use the subscripts from the molecular formula to figure out how many of each type of atom you have in the compound.

Remember if no subscript is written, it is assumed to be 1, so… Looks like we have…



Step 4: Math

4a. Multiply the atomic mass by the subscript to find the total mass of each element present in the compound:

Mass of Mg: 24.3  X 1 = 24.3

Mass of Cl235.5 X 2 = 71.0

4b. Add up the mass of each element in the compound to find the total mass of the compound.

Molecular Mass of MgCl2 = Mass of Mg + Mass of Cl2

Molecular Mass of MgCl2 = 24.3 + 71.0
Molecular Mass of MgCl2 = 95.3 g/mole

Step 5: You are done. That's the answer.

Here's what that looks like written by hand:

Looks like it was written by a 3rd grader…


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