Finding Atomic Mass using Vapour Pressure

This experiment is designed to accurately determine the atomic mass of an element by measuring its vapor pressure at different temperatures. It is by far one of the most accurate and dependable methods for determining and confirming an element’s atomic mass.

Measuring Vapour Pressure to find the Atomic Mass of an Element

This experiment involves careful measurements, data analysis, and interpretation of the results using thermodynamic principles.

Materials

Samples

Any of these materials / samples could be used:

• Solid elements: A sample of copper metal or aluminum metal can be used for this experiment.
• Liquid elements: A sample of mercury or bromine can be used for this experiment.
• Gaseous elements: A sample of oxygen gas or nitrogen gas can be used for this experiment.

Equipment

Manometer

Procedure

This is the procedure to determine the atomic mass of an element:

• Prepare a pure sample of the element. The sample can be in any form, such as a solid, liquid, or gas.
  • Please Note: Impurities or contaminants may affect the vapor pressure measurements.
• The sample is placed in a closed vessel and heated to a high temperature until it vaporizes. This is done to prevent the escape of the vapour.
• The vapour pressure of the sample is measured by placing the closed vessel containing the sample in a vacuum chamber. The vacuum chamber is equipped with a manometer, which measures the pressure of the vapour.

Results / Calculations

• The atomic mass of the element is calculated using the ideal gas law, which relates the pressure, volume, temperature, and number of moles of a gas.
• The equation is PV = nRT, where P is the pressure of the gas, V is the volume, n is the number of moles, R is the universal gas constant, and T is the temperature in Kelvin.
• The number of moles of the gas is calculated from the ideal gas law and the atomic mass of the element is determined using the formula:
• Atomic mass = (mass of the sample / number of moles) * Avogadro’s number
• where Avogadro’s number is a constant (6.022 × 10²³) representing the number of atoms in one mole of a substance.

Compare the calculated atomic mass with the known atomic mass of the element. If the values are similar, then the result is considered accurate and reliable. However, if there is a significant difference, then the procedure is repeated with a new sample and using different techniques to identify any possible sources of error.

Related Post: Determining Atomic Number using Spectroscopy

Preparation of Plots/Analysis of Data

Any of these software programs can be used to analyze data, calculate, and determine the atomic mass of the element:

• Microsoft Excel, GraphPad Prism, OriginPro, MATLAB, & LabVIEW.
• For a pure sample, plot the vapor pressure data as a function of temperature in a graph and derive the slope.
• The slope corresponds to a value of -ΔH_vap/R, where ΔHvap is the heat of vaporization of the element and R is the gas constant. The atomic mass can then be calculated using the equation:
  •          M = (RT/ -ΔHvap) * (m/M_s)
  • Where M is the atomic mass of the element, T is the temperature in Kelvin, m is the mass of the sample, and M_s is the mass of the solvent used.
• Compare the results to the known value of the atomic mass of the element to confirm the accuracy of the experiment.

Related Post: Determining the Mass Number of an Isotope

Safety Measures and Guidelines

This experiment involves the use of potentially hazardous materials and equipment. Therefore, strictly  follow these safety measures and guidelines during your laboratory engagement:

Use Safety goggles, gloves, and a lab coat during the experiment

This experiment should be conducted in a well-ventilated area

All chemicals should be handled with care, and the proper disposal of waste chemicals should be followed.

Also, all equipment should be handled with care, and any damaged or faulty equipment should be reported immediately to the instructor or the technologist in charge.

An eyewash station, a fire extinguisher, and a spill kit should be readily available in case of an emergency.

There should be proper training for all personnel involved in the lab on the use of equipment, handling of chemicals, and emergency procedures.

Some Practice Questions

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This procedure requires careful preparation of the sample, precise measurement of vapour pressure, and accurate calculations to obtain a reliable atomic mass of the element.