d
MW of NaC7H5 O2 -144.11g/mol wt = 10.06 g
mol of NaC7H5O2 = 10.06 /144.11=0.0698mol
con of sodium benzoate= 0.0698 / 2.5 = 0.0279M
benzoic acid = 0.360M
ka= 6.3x10^-5 pKa = -log Ka = -log 6.3 x10^-5 = 5- 0.79934=4.20066
pH = pKa + log[salt] / [acid] = 4.20066 +log [0.0279]/ [0.36]=4.20066+log 0.0775=4.20066- 1.11069
pH= 3.08997
when HCl is added
HC7H5O2 + H+ --------> C7H5O2-
0.9 mol 0 0.0698 mol initial
0.9 + 0.0125mol 0.0698 - 0.0125 at equlibirium
pH = pKa + log [salt] / [acid ]
pH = 4.20066+ log [0.0573]/ [0.9125]
pH = 4.20066+log 0.06279452
pH = 4.20066 -1.20207= 2.99859
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