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lecture 11asf

# lecture 11asf - Acid-Base Properties of Salts A salt is...

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Acid-Base Properties of Salts A salt is another name for an ionic compound. When such compounds dissolve in water, they dissociate to form positive and negative ions. These ions may have acid or base properties. That is, these ions may either donate protons (act as acids) or attract protons (act as bases). Negative ions often attract H + in aqueous solutions and thus function as bases. Representing a negative ion as X - , its ability to attract an H + from water is shown in the reaction: X - + H 2 O HX + OH - K b = base acid acid base The value of K b for the ion X - is related to the value of K a for its conjugate acid, HX. There is an important mathematical relationship between K a and K b for a conjugate acid-base pair. HX + H 2 O H 3 O + + X - K a = ] HX [ ] X ][ O H [ 3 - + X - + H 2 O HX + OH - K b = ] X [ ] OH ][ HX [ - - K a K b = ] X [ ] OH ][ HX [ ] HX [ ] X ][ O H [ 3 - - - + = [H 3 O + ][OH - ] = K w K a K b = K w It is useful to compare K b expressions for a molecule such as NH 3 and an ion such as nitrite, NO 2 - . NH 3 + H 2 O NH 4 + + OH - K b = ] NH [ ] OH ][ NH [ 3 4 - + NO 2 - + H 2 O HNO 2 + OH - K b = ] NO [ ] OH ][ HNO [ 2 2 - - Note that the K b equations have the same form. When a molecule such as NH 3 attracts H + from water, a positive ion, NH 4 + , results. This ion is the conjugate acid of NH 3 . When an ion such as NO 2 - attracts an H + from water, a neutral molecule, HNO 2 results. This molecule is the conjugate acid of NO 2 - . Calculate the pH of 0.10 M NaNO 2 .

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A 0.10 M NaNO 2 solution contains Na + and NO 2 - ions. The sodium ion has neither acid nor base properties. The NO 2 - ion can attract H + ions from water and functions as a base. NO 2 - + H 2 O HNO 2 + OH - 0.10-x x x x 10 . 0 x 2 - = K b (of NO 2 - ) = ) HNO of ( K K 2 a w From the K a table, HNO 2 has K a = 7.2 x 10 -4 K b = a w K K = 4 14 10 x 2 . 7 10 x 0 . 1 - - = 1.39 x 10 -11 x 10 . 0 x 2 - = 1.39 x 10 -11 Solving: x = [OH - ] = 1.18 x 10 -6 pOH = -log(1.18 x 10 -6 ) = 5.93 pH = 14.00 - 5.80 = 8.07 The 0.10 M NaNO 2 solution is slightly basic. Calculate the pH of 0.10 M NaCN A 0.10 M NaCN solution contains Na + and CN - ions. The sodium ion has neither acid nor base properties. The CN - ion can attract H + ions from water and functions as a base. CN - + H 2 O HCN + OH - 0.10-x x x x 10 . 0 x 2 - = K b (of CN - ) = ) HCN of ( K K a w From the K a table, HCN has K a = 6.2 x 10 -10
K b = a w K K = 10 14 10 x 2 . 6 10 x 0 . 1 - - = 1.6 x 10 -5 x 10 . 0 x 2 - = 1.6 x 10 -5 Solving: x = [OH - ] = 1.3 x 10 -3 pOH = -log(1.3 x 10 -3 ) = 2.90 pH = 14.00 - 2.90 = 11.10 The 0.10 M NaCN solution is basic. Comparing the two previous examples: pH of 0.10 M NaNO 2 8.07 K a of HNO 2 7.2 x 10 -4 pH of 0.10 M NaCN 11.10 K a of HCN 6.2 x 10 -10 CN - is a stronger base (higher pH, higher [OH - ]) than NO 2 - . HCN is a weaker acid than HNO 2 . This statement makes sense based on the relationship K a K b = K w K a and K b are inversely proportional. Their product is a constant (K w ). As one goes up by a certain ratio, the other goes down by the same ratio. Note that this statement—the weaker the acid the stronger the conjugate base—is a comparative statement. It does not say that a weak acid has a strong conjugate base. Both K a and K b are small. For HNO 2 and HCN: K a (of acid) K b (of conjugate base) K a K b HNO 2 7.2 x 10 -4 1.4 x 10 -11 1.0 x 10 -14 HCN 6.2 x 10 -10 1.6 x 10 -5 1.0 x 10 -14 What is the pH of 0.10 M NaCl

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A 0.10 M NaCl solution contains Na + and Cl - ions. The sodium ion has neither acid nor base properties.
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lecture 11asf - Acid-Base Properties of Salts A salt is...

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