In: Chemistry
To 160.0 mL of a buffer solution containing 0.250 M ammonia and 0.100 M ammonium chloride, 20.00 mL of 1.00M HClO4 are added. a) Calculate the pH of the initial buffer solution. b) Calculate the pH of the new solution after the addition of acid. c) What becomes the pH if another 20.00 mL of the same acid solution is added? Pkb= 4.75
a).NH3= 160.0 ml of 0.250M
number of moles of NH3 = 0.250M x 0.160L= 0.04 moles
NH4Cl = 160.0 ml of 0.100M
number of moles of NH4Cl= 0.100M x 0.160L = 0.016 moles
PKb of NH3=4.75
for basic buffer
POH= PKb + log[salt]/[bae]
POH= 4.75 + log[0.016/0.04]
POH= 4.35
POH+ PH= 14
PH= 14-POH = 14-4.35 = 9.65
PH= 9.65
B)
HClO4 = 20.00ml of 1.00M
number of moles of HClO4 = 1.00M x 0.020L= 0.02 moles
after addition of Acid
number of moles of NH3 = 0.04 - 0.02 = 0.02 moles
number of moles of salt = 0.016 + 0.02 = 0.036 moles
POH= 4.75 + log[0.036/0.02]
POH= 5.00
PH= 14- 5.00= 9.00
PH= 9.00
c)
volume of acid = 20.00+ 20.00 = 40.00 ml
number of moles of acid = 1.0M x 0.040L = 0.04 moles
number of moles of acid = number of moles of NH3.
so it is the equivalent point
at equivalent point
PH= 7- 1/2[PKb + logC]
C = number of moles /total volume
total volume= 160.0+ 40.0= 200 ml= 0.2L
C= 0.04/0.2= 0.2M
PH= 7- 1/2[4.75 + log(0.2)]
PH= 4.97