Page 216 - Chemistry eBook
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How to Write a Formula that has a Polyatomic Ion in it
We follow the usual steps for writing a chemical formula. The only difference is that we will place the polyatomic ion in parentheses.
Write the chemical formula of calcium hydroxide.
1. List the elements’ symbols. Check Reffy, Table E (p. 2) to find out the symbol of hydroxide. We get CaOH.
2. Assign oxidation numbers and put the polyatomic ion in parentheses.
Cas oxidation number is +2 ( Reffy, Periodic Table). Find the oxidation number of OH on Reffy, Table E (p. 2). It is -1.
We write: Ca+2(OH)-1
3. Crisscross: Swap the oxidation numbers of both atoms. The subscript goes outside the parentheses.
When the subscript is one, don’t write any subscript.
4. We cannot reduce this formula, so our final answer is: Ca(OH)2.
215 © UNIT 6: MATH IN CHEMISTRY
