Problem 16 - Chemistry: Introducing Inorganic, Organic and Physical Chemistry

Question

As part of an investigation into the harmful effects of petrol engine emissions, a \(10 \mathrm{g}\) sample of grass from a roadside verge was analysed for its lead content. The grass was burned to ash in oxygen to remove organic material and the inorganic residue was dissolved in \(20 \mathrm{cm}^{3}\) of dilute acid. Under certain conditions, this gave an absorbance of 0.72 on an atomic absorption spectrometer. A standard solution containing \(1.0 \mu \mathrm{gcm}^{-3}\) of lead was available. Aliquots of this were made up to \(50 \mathrm{cm}^{3}\) with dilute acid and the absorbances measured as shown in the table. $$\begin{array}{llllll}\hline \text { Volume of standard } & 5 & 10 & 15 & 20 & 25 \\\\\text { solution in } 50 \mathrm{cm}^{3} /\mathrm{cm}^{3} & & & & & \\\\\text { Absorbance } & 0.26 & 0.52 & 0.81 & 1.04 & 1.30 \\\\\hline\end{array}$$ Calculate the concentration of lead in the grass in parts per million (by mass). (Section 11.4 )

Step-by-Step Solution

Answer

The concentration of lead in the grass is approximately 20 ppm.

Step 1: Understand the Problem

We need to determine the concentration of lead in a grass sample from a roadside verge. The grass was analyzed using atomic absorption spectrometry after being converted to an ash and then dissolved in an acid.

Step 2: Analyze the Given Data

The absorbance for the grass sample is 0.72. A standard solution with known lead concentrations provides absorbance values for different volumes of this solution.

Step 3: Establish Absorbance vs. Concentration Relationship

Use the standard solution data to create a relationship between absorbance and the lead concentration. This can be done by plotting a graph of absorbance (y-axis) against concentration (x-axis) or directly calculating a linear equation from the data.

Step 4: Calculate Concentration of Lead in the Sample Solution

Use the linear relationship derived from the standard data to find the concentration of lead in the sample solution based on its absorbance of 0.72.

Step 5: Determine Lead Concentration in Grass Sample in ppm

Convert the concentration of lead found in the sample solution to ppm. Consider the sample's mass and the total solution volume it was dissolved in.

Key Concepts

Lead Concentration

When assessing the lead concentration in a sample, it's important to determine how much lead is present relative to the mass of the sample. Lead concentration is usually expressed in parts per million (ppm), which essentially compares the weight of the lead to the total weight of the sample.
For example, if you find that a 10-gram sample of grass contains 0.00002 grams of lead, the concentration would be 2 ppm. This means for every million parts of the grass, there are 2 parts lead.
Understanding lead concentration is crucial, especially when examining its impact on the environment, as it can affect both plant and animal life.

Absorbance Measurements

Absorbance measurements are a fundamental part of atomic absorption spectrometry. This process allows for precise quantification of specific elements, like lead, within a sample.
When light passes through a sample, certain wavelengths are absorbed by the elements present. The amount of light absorbed correlates directly with the concentration of the absorbing element, which in this case is lead.
Higher absorbance values indicate higher concentrations of the element in the sample. By comparing the absorbance of the sample to a set of standard solutions, you can accurately determine the element's concentration in your sample.

Inorganic Analysis

Inorganic analysis involves examining samples to determine their inorganic compounds, which do not contain carbon-hydrogen bonds. In the context of measuring lead levels, inorganic analysis helps identify elements and quantify their concentrations in the sample.
This process often involves dissolving a sample's inorganic residue after removing organic material. In the given exercise, the grass was converted to ash to remove organic compounds, leaving behind residues like lead that require analysis.
This method creates a clearer picture of the elemental makeup of a sample, essential for understanding environmental and chemical compositions.

Environmental Chemistry

Environmental chemistry focuses on the chemical reactions and transformations occurring in the environment. This discipline plays a crucial role in assessing pollutants like lead.
Lead is a significant environmental concern because it can contaminate soil and water, posing risks to ecosystems. The roadside grass analysis showcases how lead from vehicle emissions accumulates in plant life, impacting the broader environment.

• Understanding these processes helps in devising strategies to mitigate pollution.
• It also informs policy regarding environmental protection and public health initiatives.

By analyzing contaminants through environmental chemistry, we can better manage and remediate adverse effects on the ecosystem.