It takes 75mL of a 2.5M HCl solution to neutralize 55mL of a base of unknown concentration. Calculate the concentration of an unknown basic solution.

Answer 1

This type of question is multiple steps, so it's best to remember to take it one step at a time. The important thing to remember here is that almost every acid/base reaction involves H+ and OH- reacting in a 1:1 ratio to create neutral water.

1. Step One, determine the number of mols of acid that was used. The Molarity of a solution is determined by dividing the number of mols by litres of solution (mol/L). We're going to use this relationship to figure out how many mols of acid there are. We need units to match, so we convert 75 ml to 0.075L. Then, we multiply 0.075 L by the molarity of the solution (2.5M) and get 0.1875 mols. Since we know that HCl dissociates in water, we now know that there were 0.1875 mols of H+ used in the neutralization reaction.
2. Step Two, determine the number of mols of base used. This step is easy, since acid and base react in a 1:1 ratio. So, if we have 0.1875 mols of acid (H+) we know that it had to react with 0.1875 mols of base (OH-)
3. Step Three, determine the molarity of the basic solution. Now we have mols of base, and the volume (55ml) was given in the question. Molarity is just mols divided by liters, so we have to convert 55ml to 0.055L, then divide 0.1875 mols by 0.055L to get the answer, 3.4 M!