Answer to Question #81854 in General Chemistry for Reem Al-khalil

Question

To determine the concentration of a solution of hydrofluoric acid, a 75.00-mL sample is placed in a flask and titrated with a 0.1361 M solution of sodium hydroxide. A volume of 23.90 mL is required to reach the phenolphthalein endpoint. Calculate the concentration of hydrofluoric acid in the original sample.

Answer 1

Answer:

The equation for the reaction is HF(aq) + NaOH(aq) → NaF(aq) + H2O(l).

At the equivalence point the moles of the acid (HF) are equal to the moles of the base (NaOH). We can use the known concentration and measured volume of the sodium hydroxide to find the number of moles of hydroxide used in the titration:

moles NaOH = (0.1361 mol NaOH/1 L)*(0.02390 L NaOH) = 0,00325 mol NaOH where the volume has been converted to liters.

At the equivalence point: mol HF = mol NaOH so moles of HF = 0.00325 mol The concentration of the acid is equal to the number of moles divided by its volume:

molarity of acid = 0.00325 mol HF * 0.075 L = 0.244 mol/L