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How could you determine the concentration of a specific molecule, ion or…

- - - - Spectrophotometry
        
        Spectrophotometry is a technique that is used to measure the amount of light a substance absorbs or transmits at specific wavelengths. The data collected is quantitative and can therefore be used to determine the concentration of a substance within a solution (Eyring, 2014). This is done using the Beer-Lambert Law: A=εcl, where A is the absorbance value measured using a spectrophotometer, ε is the molar absorptivity (M⁻¹cm⁻¹), c is the concentration (mol/L), and l is the length of the cuvette (usually 1 cm) (Bolic, 2014).
        
        Experiment
        
        Dependent Variable: The absorbance of each solution at the selected wavelength measured by the spectrophotometer.
        
        Independent Variable: The concentration of the standard solution of the target substance (mol/L or g/L ).
        
        Controlled Variable: Wavelength setting of the spectrophotometer (Should be maintained at λmax throughout the experiment.), Cuvette size (1 cm length), Material and type of cuvette, Amount of solution in each cuvette, Mixing methods (ensures homogeneity prior to measuring)
        
        Limitations
        
        Although spectrophotometry is a highly utilized technique for finding the concentration of a substance, it has various limitations. One limitation is that at low concentrations the change in absorbance may be too small to be detected accurately by the spectrophotometer, resulting in inaccuracies in data and calculations (Berthold, 2025). Furthermore, spectrophotometry will not work as well for substances that are cloudy or turbid, as it scatters light and therefore interferes with the readings on the spectrophotometer, leading to inaccurate calculations and data (Carolina.com, 2018).
        
        Possible errors
        
        Random
        
        Mixing the solution inconsistently may result in the uneven distribution of particles within the solution and therefore lead to variation in the light absorbance measured by the spectrophotometer, and consequently the data may lack precision and therefore impact the reliability of the results.
        
        Systematic
        
        If the spectrophotometer is not calibrated properly, all of the measured absorbance values will be consistently higher or lower by the same value across all tests. Consequently, the calcuations of concentration that follow will also be consistently higher or lower than the true value. This can compromise the accuracy and validity of the data.
      - Titration
        
        Titration is a method used to determine the concentration of an unknown solution (analyte) by reacting it with a solution that has a known concentration (titrate). This involves the gradual addition of a standard solution of titrating reagent, or titrate, from a burette (Khan Academy, 2025). The addition is stopped when the moles of titrant (a solution with a known concentration that reacts with the analyte) added are equal to the moles of analyte (the solution of which you want to determine the concentration of) and the equivalence point is reached (Britannica, 2024). The volume of the titrate is then used in order to calculate the concentration of the substance.
        
        Experiment
        
        Controlled Variables: Type of base and acid being used, Concentration of the base being used, indicator used, and number of drops, Volume of acid measured, Method of swirling the flask.
        
        Independent Variable: Volume of the base added
        
        Dependent Variable: Volume of acid required to reach the endpoint or calculated concentration of the acid based on titration data.
        
        Limitations
        
        A limitation of titration is that the endpoint is determined by a change in color, which is a visual change and therefore subjective, meaning that it has the potential to cause inconsistent and therefore unreliable data. Furthermore, side reactions may occur during titration, which can limit the accuracy of results by interfering with the main reaction between analyte and titrant (Levy, 2024).
        
        Possible Errors:
        
        Systematic
        
        Not calibrating the burette will result in the burette consistently giving too much or too little liquid; consequently, the volume readings will be inaccurate in every trial by a set but unknown amount, therefore ruining the accuracy and validity of the experiment.
        
        Random
        
        Inconsistent swirling of the flask will cause the titrant and analyte to not be mixed evenly throughout all trials. This error will cause each trial to reach the endpoint at a different volume, introducing variability into results, reducing the precision and reliability of the results.
        
        Formula: M₁V₁ = M₂V₂
        Where M₁ is the molarity (concentration) of the titrant (solution of known concentration).
        V₁ is the volume of the titrant used.
        M₂ is the molarity (concentration) of the analyte (solution of unknown concentration).
        V₂ is the volume of the analyte.
      - Chromatography
        
        Chromatography is a technique used to differentiate between and separate varying components of mixtures through a stationary and mobile phase. The stationary phase stays fixed in place while the mobile phase carries the components of the mixture across the medium being used. Due to the differences in the characteristics, such as solubility, of some components, they move at different speeds across the medium, leading to the separation of the substances within the mixture (Libretexts, 2013).
        
        Experiment
        
        Limitations
        
        Although chromatography can be a useful technique for separating and identifying substances within a mixture, it is limited by its inability to provide accurate quantitative data and therefore does not allow for precise calculation of the concentration of specific substances within a mixture. Furthermore, substances with similar Rf values may overlap, making it difficult to identify and estimate the concentration of a specific one (Sawikowska et al., 2021).
        
        Possible Errors:
        
        Systematic
        
        The use of an impure solvent affects the interaction between components and the mobile phase in every trial and therefore shift the Rf value equally across all samples, therefore reducing the accuracy of the experiment (Study Mind, 2024).
        
        Random
        
        Chromatography paper may not be lying flat on an even surface or may even be bent, leading to variation in capillary action and therefore affecting the overall precision of the results (Sciencedirect, n.d.).
        
        Dependent Variable: The distance travelled by each component divided by the distance travelled by the solvent (Rf value).
        
        Independent Variable:The substance being tested (e.g., different food dye mixtures)
        
        Controlled Variables: Size and type of chromatography paper, Type of solvent (water), Volume of solvent, Temperature, Time given for the substance to absorb in the paper, Starting position of substance on the paper.
      - Gravimetry
        
        Gravimetric analysis is a method that is used to determine the amount of a specific analyte in a sample by converting it into a known substance that allows it to be weighed. The process is initiated by a precipitation reaction, where a precipitating agent is added to the solution consisting of the analyte to form a solid precipitate. The next step is digestion, which causes precipitate particles to become enlarged, preparing them for filtration. After the solids have been separated from the liquids during filtration, they are washed and dried to enhance purity and reduce variation in results. Then they are weighed, and the concentration can be calculated using the mass of the analyte and the mass of the original solution (Rohland, 2022).
        
        Experiment
        
        Independent Variable: The concentration of analyte in the original solution (g/L or mol/L).
        
        Dependent Variable: Mass of dried precipitate (g)
        
        Controlled Variables: Starting volume of the solution, Volume of precipitating agent, Method of purifying the precipitants, Stirring method, Drying temperature.
        
        Possible Errors
        
        Systematic
        
        Impurities in the tap water can react with the analyte or precipitating agent and therefore cause variation in the amount of precipitate that would otherwise be formed.
        
        Random
        
        The amount of precipitate lost during filtration can vary from one time to another, therefore causing variability in the measured mass and ultimately decreasing the precision, reliability, and overall reproducibility of the experiment.
        
        Limitations
        
        A limitation of gravimetric analysis is that it cannot be applied to all substances, as some of them may not have a suitable precipitating agent associated with them, therefore not allowing for accurate results to be obtained. Furthermore, gravimetric analysis is a very time consuming method and can take several hours, making it unsuitable for acquiring quick results.