# Modify equation 66 for bases k b c o 5 012 100 12

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Modify Equation 6.6 for bases. % = K b c o × 100% = 1.8 × 10 -5 0.12 × 100% = 1.2% (less than 5% so assumption valid) 75. What are the concentrations of all P containing species in 0.084 M K 3 PO 4 ? What is the pH of the solution? 1) PO 4 3- + H 2 O U HPO 4 2- + OH 1- K = K w /K a =1.0x10 -14 /4.8x10 -13 = 0.021; Let x = [HPO 4 2- ]=[OH 1- ] & 0.084-x = [PO 4 3- ] x 2 0.084-x = 0.021; x 2 + 0.021 x - 1.75 × 10 -3 = 0 x = 0.021 + (0.021) 2 -4(1)(-1.75 × 10 -3 ) 2(1) = 0.033 M = [OH 1- ] = [HPO 4 1- ] [PO 4 3- ] = 0.084 - 0.033 = 0.051 M 2) HPO 4 2- + H 2 O U H 2 PO 4 1- + OH 1- K 2 = K w /K a =1.0x10 -14 /6.2x10 -8 = 1.6x10 -7 Small K, so assume x is negligible, which means that [H 2 PO 4 1- ] = K 2 = 1.6x10 -7 M 3) H 2 PO 4 2- + H 2 O U H 3 PO 4 + OH 1- K 3 =1.0x10 -14 /7.5x10 -3 = 1.3x10 -12 ; [OH 1- ] = 0.033 M & [H 2 PO 4 1- ] = 1.6x10 -7 M 1.3 × 10 -12 = [H 3 PO 4 ](0.033) 1.6 × 10 -7 ; [H 3 PO 4 ] = (1.3 × 10 -12 )(1.6 × 10 -7 ) 0.033 = 6.5 × 10 -18 M pOH = -log(0.033) = 1.48; pH = 14.00 – 1.48 = 12.52 77. Define the term salt. Given an example of a neutral, a basic, and an acidic salt. A salt is an ionic compound formed in an acid-base reaction. Neutral: KClO 4 ; Basic: KCN; Acidic: NH 4 NO 3 . 79. Indicate whether each of the following is an acidic, a basic, or a neutral salt. a) K 2 SO 4 neutral b) K 2 SO 3 basic c) KHSO 4 acidic d) K 2 CO 3 basic e) KOCl basic f) NH 4 OCl basic (K b > K a ) 81. What is the pH of each of the following salt solutions? a) 0. 12 M NH 4 Cl NH 4 1+ is acidic, while Cl 1- is neutral, so NH 4 Cl is an acidic salt: 1+ 10 -6 3 [H O ] = (5.6 10 )(0.12) = 8.2 10 M; pH = 5.09 × × b) 0.096 M KCN K 1+ is neutral, but CN 1- is basic, so KCN is a basic salt: -14 -5 b -10 1.0 10 K = = 2.5 10 4.0 10 × × × 1- -5 -3 [OH ] = (2.5 10 )(0.096) = 1.55 10 M; pOH = 2.81; pH = 11.19 × × c) 0.10 M KHSO 3 K 1+ is neutral, and HSO 3 1- is amphiprotic, so use Equation 6.12. pH = ½(pK 1 + pK 2 ) = ½(1.82 + 7.00) = 4.41
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