How To Determine A Chemical Formula
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1
2HOW TO DETERMINE A CHEMICAL FORMULA
DOES THE NAME CONTAIN THE WORD “ACID”?
A. Does the name start with “hydro-“? (It’s a binary acid.)
Determine what element is present besides hydrogen, and what ion that element forms.
Formula then follows IONIC RULES below, using hydrogen and the other element.
Examples: Hydrochloric acid is HCl; hydrobromic acid is HBr;
Hydrosulfuric acid is H
S.
2
Special case: Hydrocyanic acid is HCN
B. Other acids, containing polyatomic ions:
If the name of the acid ends in “-ic”, then look for a polyatomic ion whose name
ends in “-ate”. (If the prefix “per-“ is present, you must include it.)
Follow the IONIC RULES using hydrogen and the polyatomic ion.
If the name of the acid ends in “-ous”, then look for a polyatomic ion whose name
ends in “-ite”. (If the prefix “hypo-“ is present, you must include it.)
Follow the IONIC RULES using hydrogen and the polyatomic ion.
DOES THE NAME END IN “-ATE” OR “-ITE”?
C. A polyatomic ion is present. Look up the ion and follow the IONIC RULES below.
2-
NOTE: polyatomic ions with unusual names include dichromate (-Cr
O
),
2
7
-
-
hydrogen carbonate (-HCO
), and hydrogen sulfate (-HSO
).
3
4
DOES THE NAME CONTAIN NUMERIC PREFIXES LIKE “MONO-“, “DI-“, or “TRI-“?
D. Follow the rules for COVALENT COMPOUNDS:
An element without a prefix, or with prefix “mono-“, occurs only once (no subscript)
Use subscripts for other elements, as follows:
di- = 2; tri- = 3; tetra- = 4; penta- = 5; hexa- = 6; hepta- = 7; octa- = 8, etc.
NONE OF THE ABOVE APPLY; NAME ENDS IN “-IDE”
E. Special cases: “cyanide” and “hydroxide” are polyatomic ions; follow the IONIC RULES.
F. Ionic compounds of two elements:
Follow the IONIC RULES on the reverse side.
G. These compounds have special names that you should know:
H
O water
NH
ammonia
CH
methane
2
3
4
IONIC RULES are on the REVERSE SIDE
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