Chemistry
posted by Cody .
What is the concentration of HCl when 25.0 mL of HCl completely reacts with 10.0 mL of 0.1M solution of NH4OH (subscript 4)? Solution steps would be helpful so I can solve other similar problems.

Also, I was looking for the concentration of HCl, not the volume. The equation is:
HCl (aq) + NH4OH (aq) > NH4Cl (aq) + H2O (l). 
From the equation, we conclude that one mole of HCl will react with one mole of NH_{4}OH, which means that you only have to equate the product of the molarity of the solutions and the respective volumes, i.e.
conc.HCl*25 ml = 0.1M * 10 ml
conc.HCl
= 0.1M*10ml/25ml
= 0.04M
If the ratio of the reactants is not 1:1, an adjustment will be required. 
Thanks much! I kept getting 0.4M, but realized that I was multiplying 1M * 10 mL instead of 0.1M.