A eudiometer is a laboratory device that measures the change in volume of a gas mixture following a physical or chemical change. Depending on the reaction that is being measured, the device can take on a variety of forms. In general, it is similar to a graduated cylinder, and is most commonly found in two sizes: 50 mL and 100 mL. It is 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. Applications of a eudiometer include the analysis of gases and the determination of volume differences in chemical reactions. To use a eudiometer, it is filled with water, inverted so that its open end is facing the ground (while holding the open end so that no water escapes), and then submersed in a basin of water. A chemical reaction 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 reactant 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 experimenter may accurately read the volume of the gas produced at any given time. Normally a person would read the volume when the reaction is completed. This procedure is followed in many experiments, including an experiment in which one experimentally determines the Ideal gas law constant R. The eudiometer is similar in structure to the meteorological barometer. 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.
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