lec4_2011 - Solution that contains more than one types of...

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The ionic strength of the solution equals the sum of the ionic strength of each individual compound. Example: Calculate the ionic strength of a solution that contains 0.050 mol kg -1 K 3 [Fe(CN) 6 ](aq), 0.040 mol kg -1 NaCl(aq), and 0.03 mol kg -1 Ce(SO 4 ) 2 (aq). Solution: I(K 3 [Fe(CN) 6 ]) = ½( 1 2 *(0.05*3) + (3) 2 *0.05 + (-1) 2 *(0.05*6)) = 0.45; I(NaCl) = ½(1 2 *0.04 + (-1) 2 *0.04) = 0.04; I(Ce(SO 4 ) 2 ) = ½(4 2 *0.03 + (-2) 2 *(2*0.03)) =0.36; So, I = I(K 3 (Fe(CN) 6 ]) + I(NaCl) + I(Ce(SO 4 ) 2 ) = 0.45 + 0.04 + 0.36 = 0.85 Solution that contains more than one types of electrolytes
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Calculating the mean activity coefficient Example : Calculate the ionic strength and the mean activity coefficient of 2.0 mmol kg -1 Ca(NO 3 ) 2 at 25 o C. Solution: ionic strength I I = ½(2 2 *0.002 + (-1) 2 *(2*0.002)) = 3*0.002 = 0.006; plug I into the Debye-Huckel limiting equation log(γ ± ) = - |2*1|*A*(0.006) 1/2 ; = - 2*0.509*0.0775; = -0.0789; γ ± = 0.834;
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Experimental test of the Debye-Hückel limiting law
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Accuracy of the Debye-Hückel limiting law Example : The mean activity coefficient in a 0.100 mol kg -1 MnCl 2 (aq) solution is 0.47 at 25 o C. What is the percentage error in the value predicted by the Debye-Huckel limiting law? Solution: First, calculate the ionic strength I = ½(2 2 *0.1 + 1 2 *(2*0.1)) = 0.3 to calculate the mean activity coefficient.
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