In a titration of 25.00 mL of A solution, 32.55 mL of 0.7859 M B solution are required to reach the end-point. What is the concentration of the 25.00 mL of A?

Question

A + 2B → 2C + D

Answer 1

Remember that the endpoint of a titration occurs when as many moles of one reactant are added so as to completely react with a given amount of a second reactant; i.e., the reaction has gone 100% to completion with no limiting reagent. As such, with this particular problem, we need to find the number of moles of A before we can find the concentration of A. We can do this by converting our volume of B solution into moles of A - don’t forget that Molarity (M) is defined as mol/L.

32.55 mL B * 1 L B/1000 mL B * 0.7859 mol B/1 L B * 1 mol A/2 mol B = 0.01279 mol A

To find the final concentration of A, divide the number of moles of A by the volume of A solution (in L, don’t forget to convert!) that we started with.

0.01279 mol A/0.025 L A = 0.5116 M A