Question

Perchloric acid is a strong acid with the chemical formula HCIO4. Youdissolved a certain amount of perchloric acid in 250mL of water. you measure thepH of the solution and obtain a pH of 1.28.a) Write the reaction of perchloric acid with water.b) What is the concentration of H3O+ ions in the acid solution?c) What mass of perchloric acid was used to prepare the solution?

Answer 1

$HClO_{4(s)}+H_2O\rightarrow H_3O_{(aq)}^++ClO_{4(aq)}^-$

B) To determine the concentration from pH we use the following formula:

$\begin{gathered} pH=-log[H_3O^+] \\ 1.28=-log[H_3O^+] \\ [H_3O^+]=10^{-pH} \\ [H_3O^+]=10^{-1.28} \\ [H_3O^+]=0.052\text{ }molL^{-1} \end{gathered}$

C) Determining the mass of perchloric acid used to prepare the solution, we must first determine the number of moles in the solution.

$\begin{gathered} _nHClO_4=\frac{0.052\text{ }mol\times250\text{ }mL}{1000\text{ }mL} \\ _nHClO_4=0.013\text{ }mol \end{gathered}$

Mass is therefore:

$\begin{gathered} mass=\frac{moles}{molar\text{ }mass} \\ \\ mass=\frac{0.013mol}{100.46gmol^{-1}} \\ mass=0.000129g \end{gathered}$

Mass prepared is 1.29e-3 g.

$mass=1.29\times10^{-3}g$