How would you prepare a 50mL buffer at pH = 8.90 given the following materials: 100mL volumetric flask, 100mL beaker, an unlimited supply of deionized water, 1.0M NH3, 0.50M NH4Cl, 0.15M CH3COOH, 0.10M NaOH, 0.10M HCl, a stir bar, a calibrated pH probe, a burette and a lab quest. include relevant volumes/masses, type of glassware used, significant figures and units.
To form buffer of pH 8.90, ammonia and NH4Cl is to be used.
pOH of required buffer = 14 - 8.90 = 5.10
Kb of ammonia = 1.8 x 10-5
pKb = -log(1.8 x 10-5) = 4.75
According Henderson's equation for weak base and its salt
pOH = pKb + log[NH4Cl]/[NH3]
5.10 = 4.75 + log[NH4Cl]/[NH3]
100.35 = 2.24 = [NH4Cl]/[NH3]. (Equation 1)
Hence, ratio of [NH4Cl] : [NH3] is 2.24 : 1
To get that ratio of concentrations, let the volume of 0.50 M NH4Cl to be added is V ml
Required volume of buffer = 50 ml
Hence, effective concentration of NH4Cl is (V x 0.50)/50
Volume of 1.0 M NH3 = (50 -V ml)
Effective concentration of NH3 = {(50-V) x 1.0}/50
Substituting the values of effective concentrations in equation 1
2.24 = (V x 0.50)/{(50-V) x 1.0}
2.24(50-V) = 0.50 V
112 - 2.24 V = 0.50 V
V = 112/2.75 = 40.9 ml
So, 40.9 ml of 0.50 M NH4Cl solution should be taken in a 100 mL volumetric flask using a burette.
To it (50-40.9) i.e. 9.1 ml of 1.0 M NH3 solution should be added using another burette.
Stir the solutions and stopper the flask and label the solution as buffer of pH = 8.90
How would you prepare a 50mL buffer at pH = 8.90 given the following materials: 100mL...
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