Mastering Chemistry: 9. Ch 16a HW Google Chrome https:// masteringchemistry.com
ID: 901916 • Letter: M
Question
Mastering Chemistry: 9. Ch 16a HW Google Chrome https:// masteringchemistry.com 55537070 Session General Chemistry II 9. Ch 16a HW Problem 16.59 9.2 M propionic acid (C2H5COOH Problem 16.59 Express your answer using two decimal places. Calculate the pH of each of the following solutions (Ka and Kb values are given in Appendix D in the textbook). pH Submit My Answers Give Up Part B 0.110 Mhydrogen chromate ion (HCro Express your answer using two decimal places. pH Submit My Answers Give Up Part C 0.119 M pyridine (C5H5N). Express your answer using two decimal places. pH M Help Close previous l 49 of 51 l next EL 5 PM 10/26/2015Explanation / Answer
part a.
Propionic acid is a weak carboxylic acid , hence Ostwald's dilution law is applicable for its ionic dissociation (ionization) yielding H+ ion
C2H5COOH <----------------> C2H5COO- + H+ ; Ka = 1.3 x 10-5
According to Ostwald's Dilution Law Ka = c x (alpha)2 [ applicable for weak acids / bases only ]
where , c is concentration of acid in moles / litre = Molar concentration & '' alpha '' represents degree of dissociation of the acid.
therefore, alpha = SQRT { Ka / c ]
Substituting given values as c = 9.2 x 10-2 & Ka = 1.3 x 10-5 we can find out the value of alpha,
alpha = SQRT { 1.3 x 10-5 / 9.2 x 10-2 ] = SQRT { 0.1413 x 10-3 } = 0.01
So since alpha represents number of moles of acid dissociated out of its one mole giving out alpha moles of H+ ion , therefore the concentratioon of H+ giveen out by dissociation of 9.2 x 10-2 M propionic acid = 9.2 x 10-2 x 0.01M
Apply this concentration of H+ to get pH value in relation pH = - log [ H+ ]
----------------------------------------------------------------------------------------- = -log (9.2 x 10-2 x 0.01 )
-------------------------------------------------------------------------------------------= 3.0362 or, = 3.04
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part b.
Similarly calculate the pH value of HCrO4- ion as this again gets weakly dissociated to give H+ with
Ka= 3.2 x 10-7 using Ostwald's dilution law ie.
alpha = SQRT [ c / Ka ] = SQRT{ 0.110 / 3.2 x 10-7 } = 5.86 x 10-5
& hence pH = -log ( 0.11 x 5.86 x 10-5 ) = 5.19
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------- * part c.
Same way find OH- concentration in case of pyridine which acts as a weak base with Kb = 1.7 x 10-9
using Ostwalds dilution law relation
alpha = SQRT { Kb / c }
and then calculate p(OH ) in this case p(OH) = -log (OH- )
Finally use the relation : pH + p(OH ) = 14 , to get pH value of the solution.
The answer is pH = 9.15
* Glad to help , but please post this ( ie Q.at part c ) separately for detailed calculations.
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