Book: Principles of Enviromental Engineering and Science 2ndedition Author: Mack

Question

Book: Principles of Enviromental Engineering and Science 2ndedition
Author: Mackenzie L. Davis and Susan J. Masten
Question: 2-13 A solution of acetic acid is prepared in water byadding 11.1 g of sodium acetate to volumetric flask and brining thevolume to 1.0L with water. The final PH is measure to be 5.25. Whatare the concentrations of acetate and acetic acid in solution(Assume that the tempature of soultion is 25 degree celceius) To be perfectly honest I have no idea how to set it up. I knowconcentration has various units but the overall generzilation isquainty of subtance/ quainty of particular phase (which im notquite sure what particular phase represents). Like I said I wish Icould say I knew what to do but it would probarly be a page rant onsomething that is totally wrong. Book: Principles of Enviromental Engineering and Science 2ndedition
Author: Mackenzie L. Davis and Susan J. Masten
Question: 2-13 A solution of acetic acid is prepared in water byadding 11.1 g of sodium acetate to volumetric flask and brining thevolume to 1.0L with water. The final PH is measure to be 5.25. Whatare the concentrations of acetate and acetic acid in solution(Assume that the tempature of soultion is 25 degree celceius) To be perfectly honest I have no idea how to set it up. I knowconcentration has various units but the overall generzilation isquainty of subtance/ quainty of particular phase (which im notquite sure what particular phase represents). Like I said I wish Icould say I knew what to do but it would probarly be a page rant onsomething that is totally wrong.

Explanation / Answer

Modified: ----------- According to Henderson's Equation , pH = pKa + log ( [salt ] /[ acid] ) = pKa + log ( [ A- ] / [ HA] ) We know that pKa of acetic acid = 4.74 Given pH = 5.25 Plug the values we get 5.25 = 4.74 + log ( [salt ] /[ acid] )             log ( [salt ] / [ acid] ) = 0.51                [salt ] / [ acid] = 10^0.51                                       = 3.2359 Molar mass of CH3COONa is , M = 12 + 3 * 1 + 12 + 2*16 + 23 =82 g Concentration of CH3COONa , C = ( mass / M 0 / Volume inL                                                       = ( 11.1 / 82 ) / 1 L                                                      = 0.1354 M   [salt ] / [ acid] = 3.2359 concentration of acetic acid in the solution , C' = conc. ofsalt / 3.2359                                                                       = C / 3.2359                                                                        =0.04183 M Since the total volume is 1L, the starting concentrationis 0.1354 M. but ,
the concentration of sodium acetate at the end of thereaction,x = the initial conc. - the conc. that wasprotonated.
                                                                                                = 0.1354 - x
[salt ] / [ acid] = 3.2359
( 0.1354 - x ) / x = 3.2359

x = 0.033 M
so the conc of acetate ion = 0.1354 - 0.033 = 0.1024 M
Conc . of acid , x = 0.033 M

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