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A 120.0 −mL buffer solution is 0.260 M in NH3 and 0.390 M in NH4Br, what...

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?

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Answer #1

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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