A 25.0 mL solution of HBr is neutralized with 22.3 mL of 0.105 M Sr(OH)₂. What is the concentration of the original HBr solution?

Answer 1

Hey Sydney -

This looks like a somewhat standard titration problem. I know some books like you to set up a balanced equation for this and use molar ratios, but I think the simplest way to think about it is this:

Equivalence in a titration happens when mols H+ = mols OH-

For a monoprotic strong acid (like HBR), the concentration of H+ ions would just equal the concentration of the acid.

For a strong base with only one OH- group, the concentration of OH- ions would again equal the concentration of the base.

So for a titration between these simplest acids+bases, the equivalence point is reached when M_aV_a = M_bV_b

HOWEVER, if your acid is diprotic or triprotic, that equation won't quite work. Same goes if your base has multiple hydroxide groups. This is how I write the equation for titrations:

(# of H+ available in the acid)*M_aV_a = (# of OH- available in the base)*M_bV_b

So for your specific molecules, you have

(1)*M_aV_a = (2)*M_bV_b

From here you can plug in and solve for the acid concentration. Hope this helps.