Need help calculate pH please

I missed a couple of days in Chemistry because I was sick and although I asked my teacher for help and looked at the book, I'm still a little confused on what to do here. Can someone please show me step-by-step or explain each step for these please? >< I'd really appreciate it!


Calcuate the pH after the following total volume of 0.2500M HCl have been added to 50.00mL of 0.1500M NaOH.
a) 0.00mL
b) 4.00mL
c) 29.50mL
d)30.00mL
e)30.50mL
f) 40.00mL

Wow, a lot of volumes

a) For the first one, pH would just be this.
-log(0.15M)=0.8239 pOH pH=14-0.8239=13.2

b) You need to do acid/base stoichiometry first
H+ + OH- yields H20
moles before 0.001 0.0075 ----
moles after 0 0.0065 ----

Concentration of OH- is 0.0065/0.054=0.12 molarity OH-
Note: the 0.054 is the TOTAL volume and it must be in liters

The answer would be 14 minus the -log of 0.12
pH=13.1

c) follow the steps above in part b

d) this one is a little tricky
H+ + OH- yields H20
moles before 0.0075 0.0075 --------
moles after 0 0 --------

pH would equal 7, because both strong acid and strong base completely reacts with each other, therefore the titration reaches equivalence point. Meaning equal molars of substances reacted.

e) H+ + OH- yields H20
moles before 0.007625 0.0075 ---------
moles after 1.25e-4 0 ---------

1.25e-4/0.0805=0.001553 molarity of H+ Remember to divide by the TOTAL volume
pH=-log(0.001553)=2.81

f) follow the step in part e



Notice that pH decreases as you titrate more mL of HCl, which makes sense. More strong acid titrated means that pH will become smaller and smaller.