What volume of a 2.5 m stock solution of acetic acid (hc2h3o2) is required to prepare 100.0 milliliters of a 0.50 m acetic acid solution?

We can use the dilution formula when preparing diluted solutions from more concentrated solutions
c1v1 = c2v2
where c1 is concentration and v1 is volume of the concentrated solution
and c2 is concentration and v2 is volume of the diluted solution to be prepared 
substituting the values in the equation 
2.5 M x V = 0.50 M x 100.0 mL 
V = 20 mL 
20 mL of 2.5 M stock solution is required to prepared the diluted solution

It is called a neutralization reaction