One of the tripping points for many students in acid/base problem is simply identifying compounds as acids or bases. Step 1 in the process is to memorize the strong acids and strong bases. Next you need to be able to recognize the weak acids and bases. One of the best ways to do this is to learn something about the names and the structures of the major classes of acid/base compounds. Note we are pairing up compounds based on structures and looking at the conjugate acid base pairs.
Carboxylic acids and their conjugate bases are among the most important compounds to be able to recognize as they are extremely important in organic and biochemistry.
Carboxylic acids have a carboxylic acid group, a carbon double bonded to an oxygen also bonded to an oxygen with a hydrogen. It is this hydrogen that is the acidic hydrogen. When the proton is removed, the remaining ion is called a carboxylate.
This is shown below for acetic acid.
You can identify the compounds either by their chemical structure or their name. When written out, the formula for a carboxylic acid is given as COOH and a carboxylate as COO-. The names also follow these same patterns. The acid has the name 'something-ic acid.' The conjugate base has the name 'something-ate.'
Here a few carboxylic acid names along with the names of their conjugate base ions.
formic acid, HCOOH the formate ion HCOO-
acetic acid, CH3COOH the acetate ion CH3COO-
benzoic acid, C6H5COOH the benzoate ion C6H5COO-
The next class of compounds that is important to recognize are amines and their conjugate acids. These compounds are essentially derivatives of ammonia (NH3) and its conjugate acid the ammonium ion (NH4+).
These compounds generally have the name 'something- amine' or 'something ammonium.'
They can be recognized by the chemical formula which will look like ammonia with either one hydrogen substituted for a carbon chain (primary), two hydrogens substituted (secondary), or three hydrogens substituted (tertiary). The corresponding conjugated acids will be the same, but with an added hydrogen and a positive charge
ammonia, NH3 the ammonium ion, NH4+
methyl amine , CH3NH2 the methyl ammonium ion, CH3NH3+
methyl ethyl amine, CH3NHC2H5 the methyl ethyl ammonium ion, CH3NH2C2H5+
Below is a picture of the weak base dimethyl amine accepting a proton to form the dimethylammonium ion.
The next class of compounds are hydracids. These are acids in which the proton is combined with an anion. In this case the name of the compound is 'something acid' and the corresponding conjugate base is simply the name of the anion that forms from deprotonation. For example there are several strong acids that are hydracids. The hydrochloric acid, HCl, has a corresponding conjugate base the chloride ion, Cl-. Since HCl is considered to be infinitely strong, Cl- is infinitely weak as a base. However, there are other compounds like hydrofluoric acid, HF and hydrocyanic acid HCN in which the conjugate acids anions are weak bases. In the case of HF, the conjugate base is the fluoride ion, F-. For cyanic acid, the weak base is then cyanide ion, CN-.
Finally there are the oxoacids. These are protonated non-metal oxides. For example nitric acid, HNO3. The corresponding conjugate base is nitrate, NO3-. (like chloride since the acid is infinitely strong, the conjugate base is infinitely weak). There is a systematic way to name these acids and their corresponding conjugate base anions. First it is most important to be able to recognize them. The acids are non-metals bonded with oxygen and hydrogen. The conjugate bases are identical, but they are missing a proton (minus one hydrogen and they are anions).
Chlorous Acid, HClO2 the chlorite ion, ClO2-
Chloric Acid, HClO3 the chlorate ion, ClO3-
Carbonic Acid, H2CO3 the hydrogen carbonate ion, HCO3_
What is the conjugate acid of ammonia NH3?
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