Problem 128 - Chemistry and Chemical Reactivity

Question

Thioridazine, \(\mathrm{C}_{21} \mathrm{H}_{26} \mathrm{N}_{2} \mathrm{S}_{2},\) is a pharmaceutical agent used to regulate dopamine. (Dopamine, a neurotransmitter, affects brain processes that control movement, emotional response, and ability to experience pleasure and pain.) A chemist can analyze a sample of the pharmaceutical for the thioridazine content by decomposing it to convert the sulfur in the compound to sulfate ion. This is then "trapped" as water-insoluble barium sulfate (see Figure 4.4). $$\begin{aligned}&\mathrm{SO}_{4}^{2-}(\mathrm{aq}, \text { from thioridazine })+\mathrm{BaCl}_{2}(\mathrm{aq}) \rightarrow\\\&\mathrm{BaSO}_{4}(\mathrm{s})+2 \mathrm{Cl}^{-}(\mathrm{aq}) \end{aligned}$$ Suppose a 12 -tablet sample of the drug yielded \(0.301 \mathrm{g}\) of \(\mathrm{BaSO}_{4} .\) What is the thioridazine content, in milligrams, of each tablet?

Step-by-Step Solution

Answer

Each tablet contains 40.42 mg of thioridazine.

Step 1: Find Moles of BaSO4

First, calculate the moles of \( \mathrm{BaSO}_4 \) using its molar mass. The molar mass of \( \mathrm{BaSO}_4 \) is approximately 233.39 g/mol. Use the equation: \[ \text{moles of } \mathrm{BaSO}_4 = \frac{0.301 \text{ g}}{233.39 \text{ g/mol}} \].

Step 2: Find Moles of SO4^2-

From the reaction equation, it is clear that 1 mole of \( \mathrm{BaSO}_4 \) corresponds to 1 mole of \( \mathrm{SO_4^{2-}} \). Thus, the moles of \( \mathrm{SO_4^{2-}} \) are the same as the moles of \( \mathrm{BaSO}_4 \). Use the moles calculated in Step 1.

Step 3: Find Moles of Thioridazine

Each mole of \( \mathrm{C}_{21}\mathrm{H}_{26}\mathrm{N}_{2}\mathrm{S}_{2} \) contains 2 moles of sulfur, hence 2 moles of \( \mathrm{SO_4^{2-}} \). Therefore, the moles of thioridazine are half the moles of \( \mathrm{SO_4^{2-}} \) calculated in Step 2.

Step 4: Calculate Mass of Thioridazine

Using the molar mass of thioridazine (\( \mathrm{C}_{21}\mathrm{H}_{26}\mathrm{N}_{2}\mathrm{S}_{2} \)), which is approximately 370.58 g/mol, calculate the mass: \[ \text{mass of thioridazine} = \text{moles of thioridazine} \times 370.58 \text{ g/mol} \].

Step 5: Find Thioridazine Content Per Tablet

Divide the mass of thioridazine by the number of tablets (12) to get the mass of thioridazine per tablet in grams. Then, convert this mass into milligrams by multiplying by 1000. \[ \text{mass per tablet in mg} = \left( \frac{\text{mass of thioridazine in g}}{12} \right) \times 1000 \].

Key Concepts

Chemical Analysis

Chemical analysis is a technique used to determine the composition of materials. In the context of pharmaceuticals, it's essential for ensuring the proper dosage and efficacy of drugs. To understand the exercise, consider the process of determining the thioridazine content via chemical analysis. The chemist decomposes the sample so that the sulfur component is turned into a sulfate ion. This can then be measured by precipitating it as barium sulfate.

• **Purpose of Conversion:** This conversion to barium sulfate is key because it allows for easy measurement. Barium sulfate is insoluble in water, making it straightforward to separate and weigh.
• **Analysis Method:** Techniques like gravimetric analysis, where a substance is converted to a specific compound that is weighed, are applied. These methods ensure precise and accurate determination of substance concentration.
• **Reaction Understanding:** The reaction of thioridazine-derived sulfate with barium chloride forms a solid precipitate of barium sulfate, allowing clear measurement.

Pharmaceutical Chemistry

Pharmaceutical chemistry focuses on drug design and testing, ensuring medications are safe and effective. In this exercise, you see how it's crucial for determining the proper composition of a drug like thioridazine. This agent, used to regulate dopamine, requires precise formulation for safe use.

• **Drug Function:** Thioridazine acts on neurotransmitters in the brain. It's critical that drug concentrations are correct, as any deviation might impact efficacy or lead to side effects.
• **Quality Assurance:** Through reactions such as the conversion to barium sulfate, pharmaceutical chemistry plays a vital role in checking the quality and efficacy of a medication.
• **Compliance and Safety:** Ensuring that each tablet contains the exact same amount of thioridazine is vital for patient safety and drug compliance.

Molar Mass Calculation

Calculating molar mass is a fundamental part of stoichiometry. It helps in quantifying the amount of substance involved in a chemical reaction. Here, the calculation assists in determining the quantity of thioridazine present in the sample.

• **Importance of Molar Mass:** Knowing the molar mass is crucial for converting between mass and moles, a core process in many chemical analyses.
• **Example Calculations:** In the exercise, the molar mass of barium sulfate is approximately 233.39 g/mol, used to find moles from given mass. Similarly, the molar mass of thioridazine is 370.58 g/mol, used to find the mass from moles.
• **Application of Molar Mass:** These calculations show how to trace the amount of each compound involved, ensuring the accuracy of the drug's formulation.