Question

The arsenic in a 1.22-g sample of a pesticide was converted to \(\mathrm{AsO}_{4}^{3-}\) by suitable chemical treatment. It was then titrated using \(\mathrm{Ag}^{+}\) to form \(\mathrm{Ag}_{3} \mathrm{AsO}_{4}\) as a precipitate. (a) What is the oxidation state of As in \(\mathrm{AsO}_{4}{ }^{3-} ?\) (b) Name \(\mathrm{Ag}_{3} \mathrm{AsO}_{4}\) by analogy to the corresponding compound containing phosphorus in place of arsenic. (c) If it took \(25.0 \mathrm{~mL}\) of \(0.102 \mathrm{M} \mathrm{Ag}^{+}\) to reach the equivalence point in this titration, what is the mass percentage of arsenic in the pesticide?

Short answer

The oxidation state of As in AsO4^3- is +5. The compound Ag3AsO4 is called silver arsenate. The mass percentage of arsenic in the pesticide is approximately 5.21%.

Step-by-step solution

(Step 1: Determine the oxidation state of As in AsO4^3-)

To find the oxidation state of As in AsO4^3-, we need to consider the charges of the surrounding oxygen atoms. Each oxygen atom has a charge of -2, and the total charge of the ion is -3. Let's assign the oxidation state of As as x. \(x + 4(-2) = -3\) By solving for x, we get the oxidation state of As: \(x = -3 + 4(2)\) \(x = 5\) So, the oxidation state of As in AsO4^3- is +5.

(Step 2: Name the compound Ag3AsO4)

We are asked to name Ag3AsO4 by analogy to the corresponding compound containing phosphorus in place of arsenic. The compound with phosphorus in place of arsenic is Ag3PO4, which is called silver phosphate. The only difference between these two compounds is the replacement of phosphorus by arsenic. Therefore, the analogous name for Ag3AsO4 would be silver arsenate.

(Step 3: Calculate the mass percentage of arsenic in the pesticide)

To determine the mass percentage of arsenic in the pesticide, we first need to determine the moles of As in the 1.22-g sample. We can do this using the titration information provided: 25.0 mL of a 0.102 M Ag+ solution is required to reach the equivalence point. We will first calculate the moles of Ag+ by using the volume and concentration of the solution: Moles of Ag+ = Volume × Concentration Moles of Ag+ = \(0.025 L × 0.102 mol/L\) Moles of Ag+ = \(0.00255 mol\) Now, using stoichiometry, we note that the ratio of Ag+ ions to each As atom in the balanced equation is 3:1. Moles of As = Moles of Ag+ / 3 Moles of As = \(0.00255 mol / 3\) Moles of As = \(0.00085 mol\) Next, we will convert the moles of As into grams by multiplying the moles of As by the molar mass of As, which is approximately 74.92 g/mol: Mass of As = \(0.00085 mol × 74.92 g/mol\) Mass of As ≈ \(0.06367 g\) Finally, we will calculate the mass percentage of arsenic in the pesticide: Mass percentage of As = \((Mass of As / Total Mass) × 100\% \) Mass percentage of As = \((0.06367 g / 1.22 g) × 100\% \) Mass percentage of As ≈ \(5.21 \% \) The mass percentage of arsenic in the pesticide is approximately 5.21%.

Key concepts

Oxidation States in Chemical Compounds

Understanding oxidation states in chemical compounds is crucial when studying chemical reactions, particularly those involving electron transfer, like redox reactions. In compounds, oxidation states indicate the degree of oxidation of an atom. They can be thought of as 'charge bookkeeping' where each bond contributes to the total charge an atom appears to have.

For example, in the arsenic compound \(\mathrm{AsO}_{4}^{3-}\), one must identify the oxidation states of both arsenic (As) and oxygen (O). Oxygen is typically assigned an oxidation state of -2. Since there are four oxygen atoms, their combined charge is -8. To determine the oxidation state of arsenic, you would set up an equation that sums the charges to equal the overall charge of the ion, in this case, -3. By solving it, you find that arsenic has an oxidation state of +5.

Naming Chemical Compounds

Naming chemical compounds properly is important for clear communication in chemistry. The names often provide information about the composition and structure of the compound. For instance, the compound \(\mathrm{Ag}_{3} \mathrm{AsO}_{4}\) mentioned in the exercise is a salt composed of silver (Ag) and the arsenate ion (AsO4).

By analogy, if we replace arsenic with phosphorus, we get \(\mathrm{Ag}_{3} \mathrm{PO}_{4}\), which is known as silver phosphate. Using the same logic, \(\mathrm{Ag}_{3} \mathrm{AsO}_{4}\) is named silver arsenate. Names like these are often derived from the name of the nonmetal (or metalloid, in the case of arsenic) with a specific suffix to indicate the type of compound and its charge.

Stoichiometry in Chemical Reactions

Stoichiometry is essentially the math behind chemistry. It involves the calculation of reactants and products in chemical reactions and is based on the conservation of mass and the notion of moles. A balanced equation provides the proportions in which chemicals react and are formed.

In a titration, stoichiometry helps you determine the amount of a substance required to react exactly with another substance. For instance, in the titration of \(\mathrm{AsO}_{4}^{3-}\) with \(\mathrm{Ag}^{+}\), we use the 3:1 mole ratio of \(\mathrm{Ag}^{+}\) to \(\mathrm{As}\) ions from the chemical equation. This ratio indicates that three silver ions will react with each arsenic ion to form silver arsenate. By knowing the stoichiometry of the reaction and the amount of silver ions used, we can calculate the amount of arsenic present.

Molar Concentration in Titrations

Molar concentration, often simply called concentration, is a measure of the amount of a solute present in a unit volume of solution. In chemistry, it's typically expressed as moles per liter (mol/L). Titrations rely on accurate knowledge of the concentration of one solution to determine the concentration of another.

In titration problems, such as the one mentioned, you calculate the number of moles of the analyte (the substance being measured) by using the volume and the molar concentration of the titrant (the solution with a known concentration). Once you know the moles of the analyte, you can then calculate its mass and, if you know the mass of the sample being tested, determine the mass percentage of the analyte in the sample. With the correct molar concentration, you can perform accurate and precise titrations to identify the amount of substance present in a given sample.