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Fabric Stain Removal Guide

  • More Balancing of Equations

The following is one of many emails I have recieved basically asking the same question...

"I am a little confused on why when I work out the formula for Cr3+ and F- it comes out to CrF3 instead of Cr3F3.  In a sample that is shown, Al3+ and O2- comes out to be Al2O3. We have steps to go through to get the answer, but I am really confused..."

  • The Answer is shown here...

Cr ionises to Cr3+ and F ionises to F-

To neutralise the 3 positive charges on the Cr3+ will require 3 negative charges. In this case, to obtain 3 negative charges will require 3 ionised fluorines.

So, 1(Cr3+) reacts with 3(F-). Now, when they react together, the charges are neutralised, so the 3+ on the Cr and the - on the F are no longer shown. What you have now is 1Cr3F (written as Cr1F3) BUT the 1 is never written in the molecular formula, so it becomes CrF3 (which is the correct answer).

So, you must balance the positive and negative charges on the ions to cancel each other out.

In the second equation mentioned, Al ionises to Al3+ and O ionises to O2-. Multiply the number of Al3+ by 2 (2Al3+), and we now have 6+ charges for Al. Multiply the O2- by 3 (3O2-) and we now have 6- charges for O. The 6+ and 6- cancel each other out. So, it took 2 Al3+ and 3 O2- this is written as Al2O3. To balance ionic equations you are cancelling out the positive and negative charges.


  • Remember...
    The charges, whether positive or negative have nothing to do with the formula once they cancel each other out, and this could be where your confusion lies. It is the number of atoms that becomes the number written after each atom...

    • 2 Al3+ and 3 O2- is written as Al2O3

  • Example 1

Fe3+ plus Cl-
The Fe has 3+ charges, therefore it requires 3 negative charges to cancel them out. The Cl- has only one negative charge. What are you going to do? You are going need 3 (Cl-) to get those 3 negative charges.

Therefore the final formula is written as FeCl3 (1 Fe3+ reacts with 3 Cl-).

  • Example 2

Carbon Tetrachloride is CCl4
Carbon is C4+ and Chlorine is Cl- (in their ionic state)

We have 4+ charges so require 4- charges... (4+ plus 4-) = 0 or neutral. So, to get 4- charges requires 4(Cl-)

Therefore, C4+ plus 4Cl- = CCl4

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