wait you mean that the conc of na2s2o3 is 0.9M
in the mc , it is a experiment on compering the reaction rate between diff conc of na2s2o3 with standard hcl
why 10 cm^3 is required to add in the denominator of the equation on calculating conc of na2s2o3. Although the total volume is needed for us to add 10 cm^3. but take an example of 2M HCL (100cm^3)+2M NaOH (100cm^3).
if in your calculation, it may become 2X(100/(100+100))M HCl (1M HCl) reacting with 2X(100/(100+100))M Naoh (1M Naoh)
你唔觉得有D怪咩