Question

If 12.5 mL of 0.200 M NaOH was used to reach the equivalence point for in the titration of 25.0 mL of unknown weak acid, HA, the concentration of HA is a. 0.050M b. 0.100M C.0.200M d.0.400M

Answer 1

b. 0.100M

Step-by-step explanation:

The balanced chemical reaction is: NaOH + HA ⇒ H₂O + NaA

The NaOH and HA react in a 1:1 molar ratio, so at the equivalence point, the amount of NaOH added equals the amount of HA that was present in the solution.

The amount of NaOH that was added can be calculated and set equal to the amount of HA that must have been present to react with it.

n = CV = (0.200 mol/L)(12.5 mL) = 2.50 mmol NaOH = 2.50 mmol HA

Thus, there were 2.50 mmol of HA in 25.0 mL. The concentration can be calculated as follow:

C = n/V = (2.50 mmol)/(25.0mL) = 0.100 M