Arsenic acid, H3 A s O4, is a poisonous acid that has been...

Arsenic acid, $\mathrm{H}_{3} \mathrm{~A} \mathrm{~s} \mathrm{O}_{4}$, is a poisonous acid that has been used in the treatment of wood to prevent insect damage. Arsenic acid has three acidic protons. Say you take a 26.14-mL sample of arsenic acid and prepare it for titration with $\mathrm{NaOH}$ by adding $25.00 \mathrm{~mL}$ of water. The complete neutralization of this solution requires the addition of $53.07 \mathrm{~mL}$ of $0.6441 \mathrm{M} \mathrm{NaOH}$ solution. Write the balanced chemical reaction for the titration, and calculate the molarity of the arsenic acid sample.

Key Concepts

Acid?Base Titration

This technique involves the gradual addition of a titrant (here, NaOH) to a solution of an analyte (arsenic acid) until the reaction is complete, allowing for the determination of the acid's concentration. It requires careful measurement of volumes and concentrations, and the use of an indicator or a pH meter to determine the endpoint of the titration.

Reaction Stoichiometry

Stoichiometry is used to relate the amounts of reactants and products in a chemical reaction. For an acid like arsenic acid, which has three ionizable protons, the balanced neutralization reaction with sodium hydroxide is H3AsO4 + 3NaOH ? Na3AsO4 + 3H2O. This reaction establishes the mole ratio of the acid to the base, which is crucial for calculating the concentration of the acid from the volume and molarity of the base used.

Molarity

Molarity is a measure of concentration expressed in moles of a solute per liter of solution. In titration calculations, knowing the molarity of the titrant and the volume used allows for the calculation of the moles of titrant, which, via the stoichiometric ratios of the reaction, can be related back to the moles, and hence the molarity, of the analyte.

Dilution Principle

The dilution principle is important when a solution is mixed with another solvent, which changes its concentration. In this context, the sample of arsenic acid is diluted by the addition of water before titration, requiring adjustments in volume to ensure calculations account for the changed concentration. The total solution volume after dilution is used in conjunction with the moles determined from titration to calculate the original concentration of the acid.

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