Question

A solution containing 10.0 ml of the strong acid HBr has a pH of 2.62. The weak acid is titrated with 0.124 M NaOH in the presence of a pH indicator. The indicator changes colors (approximating the equivalence point) when 15.62 ml of NaOH is added. What is the original concentration in M of the HF sample?

A) 0.50 M
B) 0.75 M
C) 1.00 M
D) 1.25 M

Answer 1

The original concentration of the HF sample, use titration and stoichiometry. At the equivalence point, the moles of NaOH added are equal to the moles of HF. Calculate the moles of NaOH added, then divide by the volume of the HF sample to find the concentration.

Step-by-step explanation:

To find the original concentration in M of the HF sample, we can use the concept of titration. In this case, we have a weak acid, HF, being titrated with a strong base, NaOH. The equivalence point is reached when the weak acid is completely neutralized by the strong base, so the number of moles of NaOH added at the equivalence point will be equal to the number of moles of HF originally present in the sample.

From the information given, we know that the pH at the equivalence point is 2.62 and that it takes 15.62 ml of NaOH to reach this point. We also know that the concentration of NaOH is 0.124 M. Using these values, we can calculate the number of moles of NaOH added at the equivalence point and then use stoichiometry to find the concentration of the HF sample.

First, let's calculate the number of moles of NaOH added at the equivalence point:

N moles of NaOH = concentration x volume = 0.124 M x 15.62 ml = 0.001937 moles of NaOH

Since the weak acid is completely neutralized by the strong base at the equivalence point, the number of moles of HF in the original sample is also 0.001937 moles. Now we can find the concentration of the HF sample:

Concentration of HF = moles / volume of solution = 0.001937 moles / 10.0 ml = 0.1937 M

Therefore, the original concentration of the HF sample is 0.1937 M.