Question

# 1) Calculate the change in pH when 3.00ml of .1M HCl(aq) is added to 100 ml...

1) Calculate the change in pH when 3.00ml of .1M HCl(aq) is added to 100 ml of a buffer solution that is .1 M in NH3(aq) and .1M in NH4Cl(aq)

2)Calculate the change in pH when 3.0ml of .1 M NaOH (aq) is added to the original buffer solution

PKb of ammonia = 4.74

From Henderson Equaton

pOH = pKb + log [NH4+]/ [NH3] =4.74

moles of NH3= 0.1*100/1000= 0.1*0.1= 0.01

HCl supplements H+ and moles of HCl added= 0.1*3/1000=0.0003

The reaction is NH3+H+ ---> NH4+. There will be an increase in [NH4+] and decrease in [NH3+]

therefore [NH4+] =0.01+0.0003 =0.0103 [NH3] = 0.01-0.0003=0.0097

pOH= 4.74+log(0.0103/0.0097)=4.766, pH= 14-4.766=9.234

b) The addition of NaOH suppliments OH- and the reaction is NH3+ ,NH4+ +OH- ---> NH3+ H2O

This will increase NH3 and decrease NH4+

Moles of OH- supplimented from NaOH= 0.1*3/1000=0.0003

[NH3] =0.01+0.0003 =0.0103 [NH4+]=0.01-0.0003=0.0097

pOH= 4.74+log (0.0097/0.0103)=4.72

pH= 14-4.72=9.28

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