What is the concentration of diluted acid if 10cm3 of concentrated HCl are diluted to 100cm3 and 10cm3 diluted acid reacts with 40cm3 0.05M...

According to the information given first 10 cm^3 of HCl
was diluted to 100 cm^3 of HCl. The diluted acid reacted with 40 cm^3 of NaOH with a
concentration of 0.05 M.

Now we need to find the
concentration of the undiluted acid.

Now the concentration
of the diluted acid is 0.05 M as one molecule of HCl reacts with one molecule of NaOH,
so the dilute HCl has the same concentration as the NaOH or
0.05.

Let the concentration of the undiluted acid be C. 90
cm^3 of water is added to it to make 100 cm^3 of diluted acid. If there were M molecules
of HCl initially per cm^3, now there are M/10 molecules of
HCl.

So as the diluted acid is 0.05 M, the concentrated
acid has a molarity of 0.05*10 = 0.5M

The
molarity of the undiluted acid is 0.5M