Understanding the Calculation of Moles in Compounds: A Comprehensive Guide

Understanding how to calculate the number of moles in a given mass of a chemical compound is a fundamental concept in chemistry. This guide will walk you through the detailed steps to calculate the number of moles in various compounds, including magnesium chlorate (Mg(ClO3)2) and magnesium chloride (MgCl2). By the end of this article, you will be able to confidently perform these calculations.

Introduction to Moles and Molar Mass

A mole is a unit used in chemistry to represent a specific quantity of entities (atoms, molecules, ions, etc.). The molar mass (or molecular weight) of a substance is the mass of one mole of that substance, measured in grams per mole (g/mol).

Calculating the Molar Mass of Mg(ClO3)2

Let's start with the calculation for magnesium chlorate (Mg(ClO3)2).

Step 1: Determine the Atomic Masses

The atomic masses for the elements in the compound are as follows:

Magnesium (Mg): 24.31 g/mol Chlorine (Cl): 35.45 g/mol Oxygen (O): 16.00 g/mol

Step 2: Calculate the Molar Mass

Now, we calculate the molar mass by adding the contributions of each element:

Magnesium: 1 x 24.31 g/mol 24.31 g/mol

Chlorine: 2 x 35.45 g/mol 70.90 g/mol

Oxygen: 6 x 16.00 g/mol 96.00 g/mol

Now, add these together:

191.21 g/mol

Calculating the Number of Moles

Once we have the molar mass, we can use it to calculate the number of moles in a given mass of the compound. The formula is:

Number of moles Mass (g) / Molar Mass (g/mol)

Step 1: Given Mass and Molar Mass

For 15.0 g of Mg(ClO3)2:

Molar mass of Mg(ClO3)2 191.21 g/mol

Step 2: Calculate the Number of Moles

Substitute the values into the formula:

Number of moles 15.0 g / 191.21 g/mol ≈ 0.0784 moles

Additional Examples

Magnesium Chloride (MgCl2)

To calculate the number of moles in 15 g of magnesium chloride (MgCl2):

Molar mass of MgCl2 1 x 24.305 g/mol 2 x 35.45 g/mol 95.205 g/mol

Number of moles 15 g / 95.205 g/mol 0.158 moles

Magnesium Chlorate (Mg(ClO3)2)

Using the same method, we can verify the previously calculated result:

No. of moles 15 g / 191.21 g/mol 0.0785 moles

Magnesium Perchlorate (Mg(ClO4)2)

To calculate the number of moles for 15 g of magnesium perchlorate (Mg(ClO4)2):

Molecular mass of Mg(ClO4)2 24 2(35.45 4 x 16.00) 172.21 g/mol

No. of moles 15 g / 172.21 g/mol 0.0870 moles

Conclusion

Through the detailed steps outlined in this guide, you can now confidently calculate the number of moles in various chemical compounds, such as Mg(ClO3)2, MgCl2, and Mg(ClO4)2. Understanding these concepts is essential for a variety of applications in chemistry, from stoichiometric calculations to solution preparation.