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On the other hand, if a chemical is a weak acid its conjugate base will not necessarily be strong. Consider that ethanoate, the conjugate base of ethanoic acid, has a base splitting constant (Kb) of about 5.6 × 10 −10, making it a weak base. In order for a species to have a strong conjugate base it has to be a very weak acid, like water.
Acetic acid, CH 3 COOH, is an acid because it donates a proton to water (H 2 O) and becomes its conjugate base, the acetate ion (CH 3 COO −). H 2 O is a base because it accepts a proton from CH 3 COOH and becomes its conjugate acid, the hydronium ion, (H 3 O +). [9]
For aqueous solutions of an acid HA, the base is water; the conjugate base is A − and the conjugate acid is the hydronium ion. The Brønsted–Lowry definition applies to other solvents, such as dimethyl sulfoxide: the solvent S acts as a base, accepting a proton and forming the conjugate acid SH +.
Here, one molecule of water acts as an acid, donating an H + and forming the conjugate base, OH −, and a second molecule of water acts as a base, accepting the H + ion and forming the conjugate acid, H 3 O +. As an example of water acting as an acid, consider an aqueous solution of pyridine, C 5 H 5 N.
If, for example, water, instead of hydroxide, was used to deprotonate the carboxylic acid, the equilibrium would not favor the formation of the carboxylate salt. This is because the conjugate acid, hydronium, has a pK a of -1.74, which is lower than the carboxylic acid. In this case, equilibrium would favor the carboxylic acid.
The following reaction represents the general reaction between a base (B) and water to produce a conjugate acid (BH +) and a conjugate base (OH −): [3] + + + The equilibrium constant, K b, for this reaction can be found using the following general equation: [3]
Its conjugate base is the acetate ion with K b = 10 −14 /K a = 5.7 x 10 −10 (from the relationship K a × K b = 10 −14), which certainly does not correspond to a strong base. The conjugate of a weak acid is often a weak base and vice versa.
compounds of O, S, Se and Te in oxidation state −2, including water, ethers, ketones; The most common Lewis bases are anions. The strength of Lewis basicity correlates with the pK a of the parent acid: acids with high pK a 's give good Lewis bases. As usual, a weaker acid has a stronger conjugate base.