Problem 214 - Chemistry Matter and Change

Question

Luminol Crime-scene investigators luminol and detervisualize blood residue. Research luminol and determine its chemical formula and percent composition.

Step-by-Step Solution

Answer

Luminol's formula is C₈H₇N₃O₂, with a percent composition of C: 54.21%, H: 3.99%, N: 23.73%, O: 18.06%.

Step 1: Understand Luminol

Luminol is a chemical used in forensic investigations to detect blood traces. It reacts with the iron in hemoglobin and emits a blue glow, allowing investigators to identify residues that are otherwise invisible.

Step 2: Find the Chemical Formula of Luminol

The chemical formula of luminol is C₈H₇N₃O₂, which describes the elements and the number of each atom in a molecule of luminol. This information is essential for calculating its percent composition.

Step 3: Calculate the Molar Mass of Luminol

Calculate the molar mass of luminol by adding the atomic masses of each element in the formula: Carbon (C): 8 atoms × 12.01 g/mol = 96.08 g/mol Hydrogen (H): 7 atoms × 1.01 g/mol = 7.07 g/mol Nitrogen (N): 3 atoms × 14.01 g/mol = 42.03 g/mol Oxygen (O): 2 atoms × 16.00 g/mol = 32.00 g/mol Total molar mass = 96.08 + 7.07 + 42.03 + 32.00 = 177.18 g/mol.

Step 4: Calculate Percent Composition of Each Element in Luminol

To find the percent composition, divide the mass of each element by the total molar mass and multiply by 100:- Carbon: \( \frac{96.08}{177.18} \times 100 = 54.21\% \)- Hydrogen: \( \frac{7.07}{177.18} \times 100 = 3.99\% \)- Nitrogen: \( \frac{42.03}{177.18} \times 100 = 23.73\% \)- Oxygen: \( \frac{32.00}{177.18} \times 100 = 18.06\% \).

Key Concepts

Forensic Chemistry

Forensic chemistry plays a pivotal role in crime scene investigations, allowing scientists to uncover hidden clues and evidence. One of the exciting applications is the use of chemical substances like luminol. In forensic settings, luminol helps reveal blood traces, even if they are not visible to the naked eye. This is achieved because luminol undergoes a chemical reaction with the iron found in hemoglobin. When luminol comes into contact with blood, it emits a blue glow. Investigators use this luminescent reaction to spot blood residues that may otherwise be overlooked. Simple, yet effective, this makes forensic chemistry an invaluable tool in criminal justice.

Chemical Formula

The chemical formula of a compound is a concise way to represent the types and numbers of atoms in a molecule. For luminol, its chemical formula is C₈H₇N₃O₂. This formula indicates each molecule of luminol contains:

• 8 carbon (C) atoms
• 7 hydrogen (H) atoms
• 3 nitrogen (N) atoms
• 2 oxygen (O) atoms

Understanding the chemical formula is crucial for various calculations, such as determining molar mass or percent composition. Each element’s presence impacts the molecule's behavior and utility. Forensic professionals and chemists rely on these formulas to predict how substances might interact with others in different environments.

Percent Composition

Percent composition tells us how much of each element is present in a compound by mass. This information is useful for understanding the make-up of the compound. In luminol, the percent composition is calculated using its formula's individual atomic masses. You start by calculating how much each element contributes to the total mass of the compound. Once you know these values:

• Carbon contributes 54.21%
• Hydrogen contributes 3.99%
• Nitrogen contributes 23.73%
• Oxygen contributes 18.06%

This breakdown helps scientists understand how each element contributes to the properties and function of luminol. By mastering percent composition, one can interpret the significance of each part of a molecule's structure.

Molar Mass Calculation

Molar mass calculation is a key skill in chemistry, allowing us to understand the mass-related features of a compound. To calculate luminol's molar mass, you add up the weighted contributions of each element present in its chemical formula. Here's how it’s done step-by-step:

• Multiply the number of atoms of each element by their respective atomic masses:
• 8 carbons: 8 × 12.01 g/mol = 96.08 g/mol
• 7 hydrogens: 7 × 1.01 g/mol = 7.07 g/mol
• 3 nitrogens: 3 × 14.01 g/mol = 42.03 g/mol
• 2 oxygens: 2 × 16.00 g/mol = 32.00 g/mol

Then, by summing up these values, you obtain the total molar mass of luminol, which is 177.18 g/mol. Understanding molar mass is fundamental in chemistry as it links atomic, molecular, and macroscopic measurements, acting as a bridge for conversions between the number of atoms and the mass of the substance on earth.