Consider a solution containing 0.100 m fluoride ions and 0.126 m hydrogen fluoride. the concentration of hydrogen fluoride after addition of 5.00 ml of 0.0100 m hcl to 25.0 ml of this solution is m.

Fluorine ions reacts with Hydrogen chloride to form more hydrogen fluoride.
Therefore, moles of HCl = 0.005 l × 0.01 M = 5 ×10^-5 moles
The initial moles of Hydrogen fluoride will be;
 = 0.0126 M× 0.0250 = 0.00315 Moles
Moles of hydrogen fluoride after the addition of HCl 
= 0.00315 + 5.0× 10^-5 = 0.0032 moles 
Therefore, the concentration of Hydrogen chloride 
            = 0.0032 moles/ 0.030 L 
            = 0.107 M

It is called a neutralization reaction