Skip to main content
Ch.4 - Reactions in Aqueous Solution
Chapter 4, Problem 114

The U.S. standard for arsenate in drinking water requires that public water supplies must contain no greater than 10 parts per billion (ppb) arsenic. If this arsenic is present as arsenate, AsO43-, what mass of sodium arsenate would be present in a 1.00-L sample of drinking water that just meets the standard? Parts per billion is defined on a mass basis as ppb = (g solute / g solution) × 109.

Verified step by step guidance
1
Step 1: Understand the definition of parts per billion (ppb). It is given by the formula: \( \text{ppb} = \frac{\text{mass of solute (g)}}{\text{mass of solution (g)}} \times 10^9 \).
Step 2: Since the problem states that the arsenic concentration is 10 ppb, set up the equation: \( 10 = \frac{\text{mass of arsenic (g)}}{\text{mass of solution (g)}} \times 10^9 \).
Step 3: Assume the density of water is approximately 1 g/mL, so the mass of 1.00 L of water is 1000 g. Use this to solve for the mass of arsenic: \( \text{mass of arsenic (g)} = \frac{10 \times 1000}{10^9} \).
Step 4: Calculate the molar mass of arsenate, \( \text{AsO}_4^{3-} \), and sodium arsenate, \( \text{Na}_3\text{AsO}_4 \). Use the periodic table to find the atomic masses: As = 74.92 g/mol, O = 16.00 g/mol, Na = 22.99 g/mol.
Step 5: Use the molar mass of sodium arsenate to convert the mass of arsenic to the mass of sodium arsenate. Use the stoichiometry of the compound to relate the mass of arsenic to the mass of sodium arsenate.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Parts Per Billion (ppb)

Parts per billion (ppb) is a unit of measurement used to describe the concentration of a substance in a solution. It is defined as the mass of solute (in grams) per mass of solution (in grams), multiplied by 10^9. This means that 1 ppb corresponds to 1 gram of solute in 1 billion grams of solution, allowing for the quantification of very low concentrations of substances, such as contaminants in drinking water.
Recommended video:
Guided course
00:57
Parts per Billion (ppb)

Molar Mass of Sodium Arsenate

Sodium arsenate (Na3AsO4) is a compound that contains sodium, arsenic, and oxygen. To calculate the mass of sodium arsenate in a solution, it is essential to know its molar mass, which is the sum of the atomic masses of all the atoms in the formula. The molar mass allows for the conversion between moles of the compound and grams, facilitating the determination of how much sodium arsenate corresponds to a specific concentration of arsenate ions in the water.
Recommended video:
Guided course
02:11
Molar Mass Concept

Dilution and Concentration Calculations

Dilution and concentration calculations are fundamental in chemistry for determining how much solute is present in a solution. When dealing with concentrations like ppb, it is crucial to understand how to manipulate the dilution equation (C1V1 = C2V2) to find the mass of solute needed to achieve a desired concentration in a given volume. This concept is vital for ensuring that the concentration of harmful substances, such as arsenate, remains within safe limits in drinking water.
Recommended video:
Guided course
02:36
Dilution Calculation Example
Related Practice
Textbook Question

The arsenic in a 1.22-g sample of a pesticide was converted to AsO43- by suitable chemical treatment. It was then titrated using Ag+ to form Ag3AsO4 as a precipitate. (a) What is the oxidation state of As in AsO43-?

Textbook Question

The arsenic in a 1.22-g sample of a pesticide was converted to AsO43- by suitable chemical treatment. It was then titrated using Ag+ to form Ag3AsO4 as a precipitate. (b) Name Ag3AsO4 by analogy to the corresponding compound containing phosphorus in place of arsenic.

Textbook Question

The arsenic in a 1.22-g sample of a pesticide was converted to AsO43- by suitable chemical treatment. It was then titrated using Ag+ to form Ag3AsO4 as a precipitate. (c) If it took 25.0 mL of 0.102 M Ag+ to reach the equivalence point in this titration, what is the mass percentage of arsenic in the pesticide?

Textbook Question

Potassium superoxide, KO2, is often used in oxygen masks (such as those used by firefighters) because KO2 reacts with CO2 to release molecular oxygen. Experiments indicate that 2 mol of KO2(s) react with each mole of CO2(g). (b) Indicate the oxidation number for each atom involved in the reaction in part (a). What elements are being oxidized and reduced?

Textbook Question

Federal regulations set an upper limit of 50 parts per million (ppm) of NH3 in the air in a work environment [that is, 50 molecules of NH3(g) for every million molecules in the air]. Air from a manufacturing operation was drawn through a solution containing 1.00⨉102 mL of 0.0105 M HCl. The NH3 reacts with HCl according to: NH3(aq) + HCl(aq) → NH4Cl(aq). After drawing air through the acid solution for 10.0 min at a rate of 10.0 L/min, the acid was titrated. The remaining acid needed 13.1 mL of 0.0588 M NaOH to reach the equivalence point. (a) How many grams of NH3 were drawn into the acid solution?