You are given solutions of hcl and naoh and must determine their concentrations. you use 27.5 ml of naoh to titrate 100. ml of hcl and 18.4 ml of naoh to titrate 50.0 ml of 0.0782 m h2so4. find the unknown concentrations.

To determine the concentrations of the substances, it is important that we equate the number of equivalents of the titrant and the substance being titrated.

(1) 27.5 mL of NaOH in 100 mL of HCl
(2) 18.4 mL of NaOH to titrate 50 mL of 0.0782 M of H2SO4

Of the two items, the one that can be easily answered is concentration of NaOH. Let x be the concentration of NaOH.
       (18.4 mL)(x) = (50 mL)(0.0782 mol/L)(2 equivalents / 1 mol of H2SO4)
The value of x from the equation is 0.425 M of NaOH

Then, for the first item, 
      (27.5 mL)(0.425) = (100)(y)

The value of y from the equation is 0.116875 M

ANSWERS: The concentrations of HCl and NaOH are 0.12 M and 0.425 M. 
             

Ido not understand.