Adding very low concentrations of acid, must use ICE?

[Chem_Mod]

Question: It asks for us to calculate the change in pH that occurs with the addition of .65 x 10^-6 mol H3O+ to sample. Instead of creating another ICE diagram, can we calculate the new moles of H3O+ in the reaction, find its molarity using the new number of moles, and then take the negative log of that to find pH? Or will we have to do the ICE diagram to earn sufficient points?

[Chem_Mod]

Answer: For this question it seems as if that would be okay since the change in x is so small that the H3O+ concentration doesn't change significantly. But you should always set up the ice box for this type of problem because you will not necessarily know if x will be so small that you can disregard it.