Phosphorus pentachloride is a white solid that sublimes (vaporizes without melting) at about 100 degrees C. At higher temperatures, the PCl5 vapor decomposes to give phosphorus trichloride and chlorine.
How could gas-density measurements help to establish that PCl5 vapor is decomposing?
Gas-density measurements can help establish that PCl5 vapor is decomposing by comparing the density of the gas at different temperatures. When PCl5 decomposes, it forms phosphorus trichloride (PCl3) and chlorine gas (Cl2). Since PCl5 has a higher molecular weight than both PCl3 and Cl2, the density of PCl5 vapor will be higher than the combined density of PCl3 and Cl2.
To conduct the gas-density measurements, one can start by measuring the density of PCl5 vapor at a specific temperature, for example, 100 degrees Celsius. This can be done using a suitable apparatus such as a gas density meter or by measuring the mass and volume of a known amount of PCl5 gas. The result will provide the density of PCl5 vapor at that specific temperature.
Next, the temperature can be increased to a higher value, for instance, above 200 degrees Celsius, where the decomposition of PCl5 is expected to occur. The density of the gas can be measured again under these conditions. If the PCl5 is decomposing, the density of the gas will decrease compared to the initial measurement.
By comparing the densities obtained at different temperatures, a significant decrease in density would indicate that PCl5 vapor is undergoing decomposition. This is because the lower-density products of the decomposition, PCl3 and Cl2, contribute to the overall gas density. Therefore, a decrease in density suggests the formation of PCl3 and Cl2 gases, confirming the decomposition of PCl5.