A 100 mL sample of hydrobromic acid, HBr, is titrated to an endpoint
with 24.0 mL of 1.5 N NaOH. What is the concentration of HBr?
a. 1.4 N
b. 0.72 N
c. 3.1 N
d. 0.36 N

Hello, 

This question follows the same logic as the previous question that I
answered for you:
http://answers.google.com/answers/main?cmd=threadview&id=227189

Using the equation from that question:

V1 * C1 = V2 * C2

where V1 = volume solution 1    V2 = volume solution 2
      C1 = concentration solution 1  C2 = concentration solution 2

The acids and bases in this question are both mono-functional (ie they
have one H+ or one OH- group) so their acid/base concentration is
given by the concentration of the parent compound.  Thus, plugging in
numbers:

V1 = 100 mL  C1 = x   V2 = 24.0 mL  C2 = 1.5 N

100 mL * x = 24.0 mL * 1.5 N    and rearranging:

     24.0 mL * 1.5 N
x = ----------------   =  0.36 N   HBr
        100 mL         


Here is a nice javascript demonstration of the principles employed:
http://michele.usc.edu/java/acidbase/acidbase.html

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titration example