In: Chemistry
2 attempts left |
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1)when 0.0 mL of HCl is added
NH3 dissociates as:
NH3 +H2O -----> NH4+ + OH-
0.69 0 0
0.69-x x x
Kb = [NH4+][OH-]/[NH3]
Kb = x*x/(c-x)
Assuming x can be ignored as compared to c
So, above expression becomes
Kb = x*x/(c)
so, x = sqrt (Kb*c)
x = sqrt ((1.8*10^-5)*0.69) = 3.524*10^-3
since c is much greater than x, our assumption is correct
so, x = 3.524*10^-3 M
So, [OH-] = x = 3.524*10^-3 M
use:
pOH = -log [OH-]
= -log (3.524*10^-3)
= 2.4529
use:
PH = 14 - pOH
= 14 - 2.4529
= 11.5471
2)when 10.0 mL of HCl is added
Given:
M(HCl) = 0.23 M
V(HCl) = 10 mL
M(NH3) = 0.69 M
V(NH3) = 10 mL
mol(HCl) = M(HCl) * V(HCl)
mol(HCl) = 0.23 M * 10 mL = 2.3 mmol
mol(NH3) = M(NH3) * V(NH3)
mol(NH3) = 0.69 M * 10 mL = 6.9 mmol
We have:
mol(HCl) = 2.3 mmol
mol(NH3) = 6.9 mmol
2.3 mmol of both will react
excess NH3 remaining = 4.6 mmol
Volume of Solution = 10 + 10 = 20 mL
[NH3] = 4.6 mmol/20 mL = 0.23 M
[NH4+] = 2.3 mmol/20 mL = 0.115 M
They form basic buffer
base is NH3
conjugate acid is NH4+
Kb = 1.8*10^-5
pKb = - log (Kb)
= - log(1.8*10^-5)
= 4.745
use:
pOH = pKb + log {[conjugate acid]/[base]}
= 4.745+ log {0.115/0.23}
= 4.444
use:
PH = 14 - pOH
= 14 - 4.4437
= 9.5563
3)when 30.0 mL of HCl is added
Given:
M(HCl) = 0.23 M
V(HCl) = 30 mL
M(NH3) = 0.69 M
V(NH3) = 10 mL
mol(HCl) = M(HCl) * V(HCl)
mol(HCl) = 0.23 M * 30 mL = 6.9 mmol
mol(NH3) = M(NH3) * V(NH3)
mol(NH3) = 0.69 M * 10 mL = 6.9 mmol
We have:
mol(HCl) = 6.9 mmol
mol(NH3) = 6.9 mmol
6.9 mmol of both will react to form NH4+ and H2O
NH4+ here is strong acid
NH4+ formed = 6.9 mmol
Volume of Solution = 30 + 10 = 40 mL
Ka of NH4+ = Kw/Kb = 1.0E-14/1.8E-5 = 5.556*10^-10
concentration ofNH4+,c = 6.9 mmol/40 mL = 0.1725 M
NH4+ + H2O -----> NH3 + H+
0.1725 0 0
0.1725-x x x
Ka = [H+][NH3]/[NH4+]
Ka = x*x/(c-x)
Assuming x can be ignored as compared to c
So, above expression becomes
Ka = x*x/(c)
so, x = sqrt (Ka*c)
x = sqrt ((5.556*10^-10)*0.1725) = 9.789*10^-6
since c is much greater than x, our assumption is correct
so, x = 9.789*10^-6 M
[H+] = x = 9.789*10^-6 M
use:
pH = -log [H+]
= -log (9.789*10^-6)
= 5.0092
4)when 40.0 mL of HCl is added
Given:
M(HCl) = 0.23 M
V(HCl) = 40 mL
M(NH3) = 0.69 M
V(NH3) = 10 mL
mol(HCl) = M(HCl) * V(HCl)
mol(HCl) = 0.23 M * 40 mL = 9.2 mmol
mol(NH3) = M(NH3) * V(NH3)
mol(NH3) = 0.69 M * 10 mL = 6.9 mmol
We have:
mol(HCl) = 9.2 mmol
mol(NH3) = 6.9 mmol
6.9 mmol of both will react
excess HCl remaining = 2.3 mmol
Volume of Solution = 40 + 10 = 50 mL
[H+] = 2.3 mmol/50 mL = 0.046 M
use:
pH = -log [H+]
= -log (4.6*10^-2)
= 1.3372