Question

. What mass A chemistry graduate student is given 125. mL of a 1.30 M benzoic acid (HCH,CO2) solution. Benzoic acid is a weak acid with Ka=6.3 10 of NaC H,CO2 should the student dissolve in the HCGH,CO2 solution to turn it into a buffer with pH = 4.61? You may assume that the volume of the solution doesn't change when the NaC,H,CO, is dissolved in it. Be sure your answer has a unit symbol, and round it to 2 significant digits. x 6 ?

Answer 1

Ka = 6.3*10^-5

pKa = - log (Ka)
= - log(6.3*10^-5)
= 4.201

use formula for buffer
pH = pKa + log ([NaC6H5CO2]/[C6H5CO2H])
4.61 = 4.2007 + log ([NaC6H5CO2]/[C6H5CO2H])
log ([NaC6H5CO2]/[C6H5CO2H]) = 0.4093
[NaC6H5CO2]/1.3 = 2.5665
[NaC6H5CO2] = 3.3364
volume , V = 1.25*10^2 mL
= 0.125 L


use:
number of mol,
n = Molarity * Volume
= 3.336*0.125
= 0.4171 mol

Molar mass of NaC6H5CO2,
MM = 1*MM(Na) + 7*MM(C) + 5*MM(H) + 2*MM(O)
= 1*22.99 + 7*12.01 + 5*1.008 + 2*16.0
= 144.1 g/mol

use:
mass of NaC6H5CO2,
m = number of mol * molar mass
= 0.4171 mol * 1.441*10^2 g/mol
= 60.1 g
Answer: 60. g