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ID: 790648 • Letter: #
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NH3 is a weak base (Kb = 1.8 times 10-5) and so the salt NH4CI acts as a weak acid. What is the pH of a solution that is 0.022 M in NH 4Cl? pH = What is the fraction of association (a) for the following potassium propionate solutions? Ignore activities. The Ka of propanoic acid is 1.34 times 10-5. 2.00 times 10-1 M K(C2H5CO2) alpha = 2.00 times 10-2 M K(C2H5CO2) alpha = 2.00 times 10-11 M K(C2H5CO2) alpha = HCIO is a weak acid (Ka = 4.0 times 10-8) and so the salt NaCIO acts as a weak base. What is the pH of a solution that is 0.049 M in NaCIO? pH = You need to prepare 100.0 mL of a pH=4.00 buffer solution using 0.100 M benzoic acid (pKa = 4.20) and 0.120 M sodium benzoate. How much of each solution should be mixed to prepare this buffer?Explanation / Answer
1) NH4+ <-------> NH3 + H+ ,
at equi [NH4Cl] = 0.022-x , [NH3]=[H+] = x
Ka = Kw/Kb(NH3) = 10^-14/(1.8x10^-5) = 5.55 x 10^-10
Ka = [NH3][H+]/[NH4+] , 5.55 x10^-10 = x^2/( 0.022-x)
x = [H+] = 3.5 x10^-6 , pH = -log [H+] = -log ( 3.5 x10^-6) = 5.46
2) C2H5COO- + H2O <----> C2H5COOH + OH-
Kb = Kw/Ka = 10^-14/1.34 x10^-5 = 7.46 x10^-10 = x^2/( 0.2-x)
x = [C2H5COOH] = 1.22 x10^ -5
fraction of association = [C2H5COOh] /[C2H5COO-] = 1.22 x10^-5 /0.2 = 6.1 x10^-5
b) Kb = 7.46 x10^-10 = x^2/( 0.02-x) , x = [C2H5COOH] = 3.86 x10^-6
fraction of association = ( 3.86 x10^-6/0.02) = 1.93 x10^-4
c) 7.46 x10^-10 = x^2/( 2x10^-11 -x)
x = 1.95 x 10^-11 , fraction of associaton = ( 1.95 x10^-11/2x10^-11) = 0.975
3) Kb = Kw/Ka = 10^-14/4x10^-8 = 2.5 x10^-7
OCl- + H2O <-> HOCl + OH-
Kb = 2.5 x10^-7 = x^2/( 0.049-x)
x = 1.107 x10^-4 = [OH-] , pOH = 3.96 , pH = 14-3.96 = 10.04
4) pH = pka + log [sodium benzoate]/[benzoic acid]
4 = 4.2 + log [sodbenzoate]/[ben acid]
[sodim benzoate] = 0.631 [benzoic acid]
sodium benzoate moles = 0.631 benzoic acid moles
let V = vol of bezoic acid in liters , then 0.1 -v = vol of sodium benzoate
hence ( 0.1-v)(0.12 )= 0.631 ( 0.1 x v)
v = 0.0655 lit = 65.5 ml
hence 65.5 ml benzoic acid and 100-65.5 = 34.5 ml of 0.12 sodium benzoate are mized too get required pH
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