A 120.0 −mL buffer solution is 0.260 M in NH3 and 0.390 M in NH4Br, what mass of HCl could be handled before the pH fell below 9.00?
The buffer is made up of 0.260M NH3 and 0.390M NH4Br
For buffer the pH is calculated as
pOH = pKb + log [salt] / [base]
pKb for ammonia = 4.7
pH = 9 so pOH = 5
The HCl added will react with NH3 to give NH4Cl so it will increase the concentration of NH4+ and will decrease concentration of NH3 (let say the moles are "x"
5= 4.7 + log [0.390 +x ] / [0.260 -x ]
0.3 = log [0.390 +x ] / [0.260 -x ]
taking antilog
1.99= [0.390 +x ] / [0.260 -x ]
0.517 - 1.99x = 0.390 + x
0.127 = 2.99x
x = 0.0424 moles
mass of HCl = Moles X molecular weight of Hcl = 0.0424 x 36.5 = 1.55 grams
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