Eudiometer

A eudiometer is a device that measures the change in volume of a gas mixture following combustion or a single or double displacement reaction. It is used to measure the amount of oxygen in the air. The device has taken on a variety of forms, depending on the reaction that is being measured. The term is named from its frequent use to determine the purity of the air. In general, the device is a graduated cylinder, closed at the top end with the bottom end immersed in water or mercury. The liquid traps a sample of gas in the cylinder and the graduation allows the volume of the gas to be measured. For some reactions, two platinum wires (chosen for their non-reactivity) are placed in the sealed end so an electric spark can be created between them. The electric spark can initiate a reaction in the gas mixture and the graduation on the cylinder can be read to determine the change in volume resulting from the reaction. The use of the device is quite similar to the original barometer, except that the gas inside displaces some of the liquid that is used.

History
The Eudiometer was first pioneered by Count Alessandro Volta (1745-1827), an Italian physicist who is well-known for his contributions to the electric battery and electricity in general, which caused the volt to be named after him. Aside from its laboratory function, the eudiometer is also known for its part in the "Volta Pistol." Volta invented this instrument in 1777 for the purpose of testing the "goodness" of air, or analyzing the flammability of gases. His initial use concerned the study of swamp gases in particular. The "pistol" was filled with oxygen and another gas. The homogeneous mixture was tapped shut with a cork. A spark could be introduced into the gas chamber of the “pistol,” and possibly catalyze a reaction by static electricity. If the gases were flammable, they would explode, and increase the pressure within the gas chamber. This pressure would be too great and eventually cause the cork to become airborne. Volta's extensive studies on measuring and creating high levels of electric currents caused the electrical unit, the volt, to be named after him.

Etymology
Eudiometer comes from the Greek root eúdio(s) meaning clear or mild, which is the combination of the prefix eu- meaning good, and dios meaning heavenly, with the suffix -meter meaning measure. Because the eudiometer was originally used to measure the amount of oxygen in the air, an amount that was thought to be greater in nice weather, the root eúdio(s) appropriately describes the apparatus. Thus, a eudiometer measures the "clarity" or the "purity" of air.

Usage
To effectively use a eudiometer, one may simply fill the eudiometer with water, flip it over so that the open end is facing the ground (while holding the open end so that no water pours out), and submerse the eudiometer in a basin of water and over a chemical reaction that is taking place through which gas is created. One reactant is typically at the bottom of the eudiometer (Which flows downward when the eudiometer is inverted) and the other is suspended on the rim of the eudiometer, typically by means of a platinum or copper wire (due to their low reactivity). When the gas created by the chemical reaction is released, it should rise into the eudiometer so that the person experimenting may accurately read the volume of gas produced from the reaction at any given time, usually when the reaction is completed. Such a procedure using the eudiometer is followed in many experiments, including an experiment in which one experimentally determines the Ideal Gas Law Constant R (see below). The eudiometer is similar in structure to the meteorological barometer. A meteorological barometer is a mercury-filled device used to measure atmospheric pressure. The column, which is sealed on one end, is put open-end up into a basin of mercury. The pressure of the atmosphere on the mercury in the basin causes the mercury in the tube to either fall or rise. Similarly, a eudiometer uses water to release gas into the eudiometer tube, converting the gas into a visible, measurable amount. A correct measurement of the pressure when performing these experiments is crucial for the calculations involved in the PV=nRT equation, because the pressure could change the density of the gas.

Determining the ideal gas constant R
Because a eudiometer measures the amount of gas, a eudiometer can be used to calculate the ideal gas law constant, R. The accepted value of this constant is around 0.0821 atm·L·mol-1·K-1, or 8.314472 Pa·L·mol-1·K-1. A eudiometer can be used to measure the amount of hydrogen gas produced when magnesium and hydrochloric acid react. To do this experiment, approximately 8mL of hydrochloric acid must be poured into the bottom of the eudiometer: the rest of the tube should be filled with water. Next, one must tie a copper wire around a thin strip of magnesium (around 0.04 g). A wide-based beaker must also be filled about halfway with water. It is then necessary to turn the eudiometer upside down before quickly placing it in the beaker. The open end of the eudiometer should be submersed in the water and flush with the base of the beaker, so the hydrochloric acid does not escape. Since the hydrochloric acid is more dense than water, it will sink to the bottom of the eudiometer tube and begin to react with the magnesium strip coiled there. By recording and calculating the atmospheric pressure, temperature, volume, and number of moles of hydrogen gas, you can use the property of equality to turn the equation PV = nRT to R = (PV)/(nT) and solve for the ideal gas constant, R.

Figuring out the R constant in the PV=nRT equation
If the experiment was conducted correctly, the data collected will help in proving the R constant for the equation PV = nRT.

P = pressure (in atm)
The pressure in this experiment includes several elements. Since the eudiometer is filled with hydrogen gas and water vapor, the total pressure (determined by the barometer) will consist of vapor pressure as well as the pressure of the hydrogen gas. The vapor pressure can easily be determined by the temperature of the water in the water pan and the corresponding pressure of that temperature in mm Hg. Likewise, the pressure of H2 gas can only be determined if the total pressure (in atm) is subtracted by the vapor pressure (in atm). This calculation follows Dalton's law of partial pressure. The resulting pressure of H2 gas will be used in the calculation.

V = volume (in L)
The volume in this experiment is the volume of H2 gas that is measured using the eudiometer. Since the Eudiometer measures in mL, a conversion from mL to L will be necessary. To do this you multiply the mL by 1L/1000mL and the result is the liters of the solution.

n = amount of substance (in moles)
The reaction of this experiment is a limiting reagent reaction. When the equation is properly balanced, the mass of the Mg should be used to determine the theoretical number of moles of H 2 that can be created. Likewise, the volume of HCl acid should also be used to determine how many moles of H 2 can be made. The lesser number of moles will be used for the equation.

R = the given constant
In normal PV=nRT calculations, the R constant is given and does not need to be determined through collected data. However, the purpose of this lab is to determine the R constant through the collected data. Therefore, no numerical value will be given to the constant at this point. R, as a constant, is 0.0821 L·atm·K-1·mol-1, which we are trying to find in this experiment.

T = temperature (in K)
The temperature should be determined by placing a thermometer into the eudiometer after the reaction is complete. This may be difficult to do, for the eudiometer is placed upside down, and moving it from its unstable position might release the excess water and HCl into the water pan. This temperature should be recorded as soon as possible after the rest of the necessary values have been recorded, as the gas will cool over time, its temperature becoming closer to that of the water. Thus, the water temperature may be accurate, though directly measuring the temperature of the gas will generally yield better results. One should note that when using a Celsius or Fahrenheit thermometer, you must convert it to kelvins. You can use the equation C = 9/5(F-32) to convert degrees Fahrenheit to degrees Celsius and use the equation K = C + 273.15 to convert degrees Celsius to kelvins.


 * $$ PV/ nT =R $$