Question

The compound 2,3 -dimercaptopropanol \(\left(\mathrm{HSCH}_{2} \mathrm{CHSHCH}_{2} \mathrm{OH}\right),\) commonly known as British Anti-Lewisite (BAL), was developed during World War I as an antidote to arsenic-containing poison gas. (a) If each BAL molecule binds one arsenic (As) atom, how many As atoms can be removed by \(1.0 \mathrm{~g}\) of BAL? (b) BAL can also be used to remove poisonous heavy metals like mercury \((\mathrm{Hg})\) and lead \((\mathrm{Pb})\). If each \(\mathrm{BAL}\) binds one \(\mathrm{Hg}\) atom, calculate the mass percent of \(\mathrm{Hg}\) in a BAL-Hg complex. (An \(\mathrm{H}\) atom is removed when a BAL molecule binds an \(\mathrm{Hg}\) atom.)

Short answer

(a) \(4.85 \times 10^{21}\) As atoms; (b) 61.92\% Hg.

Step-by-step solution

Calculate the Molar Mass of BAL

First, we need to determine the molar mass of a BAL molecule \(\text{HSCH}_2\text{CHSHCH}_2\text{OH}\). The atomic masses are approximately: H = 1 g/mol, S = 32 g/mol, C = 12 g/mol, O = 16 g/mol. The formula gives us \(3 \times 12\) g/mol for carbon, \(8 \times 1\) g/mol for hydrogen, \(2 \times 32\) g/mol for sulfur, and \(1 \times 16\) g/mol for oxygen. Add them to get the total molar mass: \[\text{Molar Mass of BAL} = (3 \times 12) + (8 \times 1) + (2 \times 32) + 16 = 124\text{ g/mol}.\]

Determine Moles of BAL in 1.0 g

Using the molar mass from Step 1, convert 1.0 g of BAL to moles using the formula: \[\text{moles of BAL} = \frac{\text{mass}}{\text{molar mass}} = \frac{1.0 \text{ g}}{124 \text{ g/mol}} \approx 0.00806 \text{ mol}.\]

Calculate Number of Arsenic Atoms

Since each BAL molecule binds one arsenic atom, the number of arsenic atoms removed will be the same as the number of moles of BAL, converted to atoms using Avogadro's number: \[0.00806 \text{ mol} \times 6.022 \times 10^{23} = 4.85 \times 10^{21} \text{ As atoms}.\]

Calculate the Molar Mass of BAL-Hg Complex

To find the mass percent of \(\text{Hg}\) in the BAL-Hg complex, we need its molar mass. Remove one H atom (1 g/mol) and add one Hg atom (200 g/mol): \[\text{Molar Mass of BAL-Hg} = 124 + 200 - 1 = 323 \text{ g/mol}.\]

Calculate Mass Percent of Hg in BAL-Hg Complex

The mass percent of \(\text{Hg}\) in the BAL-Hg complex is calculated by dividing the mass of \(\text{Hg}\) by the molar mass of the complex and multiplying by 100: \[\text{Mass percent of Hg} = \left(\frac{200}{323}\right) \times 100 \approx 61.92\% .\]

Key concepts

Molar Mass Calculation

Molar mass is an essential concept in stoichiometry. It helps us understand how much a given quantity of a compound weighs. To calculate the molar mass of a compound, you need to sum the atomic masses of all the elements present in the compound according to its chemical formula.
For example, in the compound 2,3-dimercaptopropanol (BAL), \( ext{HSCH}_2 ext{CHSHCH}_2 ext{OH}\), we find the molar mass by adding the following:

• Carbon, C: \(3 \ imes 12\) g/mol
• Hydrogen, H: \(8 \ imes 1\) g/mol
• Sulfur, S: \(2 \ imes 32\) g/mol
• Oxygen, O: \(1 \ imes 16\) g/mol

Add these up to get the molar mass: \(124 ext{ g/mol}\). This calculation allows us to convert a mass of the compound into moles, which is crucial for chemical reactions and solutions involving precise amounts of substances.

Chemical Bonding

Chemical bonding is the force that holds atoms together in molecules. It involves the sharing or transfer of electrons between atoms. In the case of BAL, its structure involves specific arrangements of atoms and bonds, including single covalent bonds between carbon and hydrogen, sulfur, and oxygen atoms.
A key aspect of chemical bonding in BAL is its ability to bind with heavy metals like arsenic (As) and mercury (Hg). This binding ability stems from the presence of sulfur atoms, which have a high affinity for bonding with metal ions. When a BAL molecule encounters a metal ion, it can form a stable complex by sharing electrons, which neutralizes the toxicity of the metal.

Mass Percent Composition

Mass percent composition is used to express the concentration of an individual element in a compound. It tells us what percentage of the total mass of the compound is made up of a specific element. To calculate the mass percent of an element in a compound, use the formula:
\[\text{Mass percent} = \left(\frac{\text{mass of the element in one mole of the compound}}{\text{molar mass of the compound}}\right) \times 100\]
In the case of the BAL-Hg complex, we calculated the mass percent of mercury (Hg) as follows:

• The mass of Hg is 200 g/mol.
• The molar mass of the BAL-Hg complex is 323 g/mol.

Thus, the mass percent is computed by \(\left(\frac{200}{323}\right) \times 100\), which results in approximately 61.92%.
This information is vital in chemistry as it tells us how much of a specific element is present, which is important for reactions and formulations involving that compound.