[jay]

hey,
      i always have troubles finding the concentration of a substance when two solutions are added. ive asked my teacher for help with questions similare to the one below but still dont understand. can you help me with this one please.

problem: A 30 ml aliquot of a 0.15M NH4Cl solution is added to 20ml of a 0.18M NH4NO3 solution. What is the concentration of ammonium ion in the mixture?

Thanks heaps
 

[movies]

For any question like this, all you need to do is figure out the number of moles of whatever material and the amount of solvent in the final mixture.

To go about this, first figure out hom many moles of NH₄⁺ are in aliquot, then figure out the number of moles of NH₄⁺ are in the solution that you are adding to.  That will give you the total number of moles in the final solution, right?

Figure the amount of solution you have when the two are combined and then calculate the concentration from the number of moles and the volume of solution.