How to determine molar mass using the Victor Meyer method

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Experimental molecular weight

Victor-Meyer method English version

Purpose

The purpose of this experiment is to learn to use the Victor-Meier method to determine the approximate molecular weight of volatile substances.

This experiment illustrates one of the many applications of the ideal gas equation.

The expression of gas molecular weight can be obtained from the ideal gas equation of state

PV=nRT=gRT/M (1)

If you know that Under a certain temperature (T) and pressure (P), the volume V occupied by g grams of gas can be calculated using this equation. R is the gas constant, and n is the number of moles of gas.

The Victor-Meier method vaporizes a known amount of material to displace a corresponding volume of air in a gas tube. The air is collected above water or mercury, and its volume is measured at a known temperature and atmospheric pressure.

Instruments and drugs

The Victor Meyer method device (shown in Figure 1-1) consists of an outer jacket tube, a gasification tube, a crushing device, a measuring tube and a level ball; Victor -Meyer ball; 100°C thermometer (grading 1°C); iron frame, clamp, gas lamp, etc.; ether, chloroform, benzene, etc.

Experimental steps

Figure 1-1 shows one of the types of Victor-Meier apparatus. Although there are no standard dimensions, the vaporization tube should be of such size and shape as to prevent any escape of the vaporized sample and subsequent connection between the measuring tube and the gasification tube. Condensation occurs at the connection part. If the gas volume is measured for too long, trouble will occur due to diffusion, so due attention must also be paid to the shape and size of the tube.

As shown in this device, use the outer tube in the The steam generated by boiling water heats the gasification tube. Because we are using the ideal gas equation of state, and the actual gas does not strictly conform to this equation. For our experiment, the temperature difference between the measured substance and the outer tube is about 2°C. Heating with water vapor limits us to only measuring substances with boiling points of about 80°C or lower. Electric heating can be used or high boiling point substances can be used in outer tubes to generate higher temperatures.

When measuring, boil the water in the outer tube for a few minutes. At this time, the top of the gasification tube is open. Then put the crushing device with the sample ball hanging on the platinum ring into the gasification tube, install it and read the level of the gasification tube. Position. This horizontal plane position should be close to the top of the measuring pipe. When the horizontal plane in the level ball and the horizontal plane in the measuring pipe are at the same height, take a reading. This is to ensure that the gas pressure is equal to atmospheric pressure. When the horizontal plane position is constant, record the meniscus position and Crush the sample ball. As the liquid vaporizes, air is forced into the measuring tube. Lower the leveling ball while it vaporizes to keep the horizontal position of the measuring tube approximately equal to the horizontal position of the leveling ball. When the height of the water no longer changes, record the bend. Moon surface position. The difference between the readings at the beginning and the last horizontal position is the volume of discharged air measured at atmospheric pressure and observed temperature. If the gas volume begins to decrease after recording the maximum volume, it means that part of the sample has entered the connection of the measuring gas pipe. , and condensation occurs. If this happens, use the maximum volume in the calculation. As soon as the gas volume is determined, read the barometer reading.

Between each measurement, use a long glass Insert the tube into the vaporization tube and pump air for one to two minutes to remove all the vapor in the tube. The vapor left from the previous measurement will condense on the gas measuring tube and the connecting system, causing trouble.

Data processing

The molecular weight of the substance can be obtained by directly substituting it into equation (1). However, there is a major source of error when calculating, and its correction is difficult. This error is caused by the vapor pressure of the water in the measuring tube. caused. The question is whether to partially or completely subtract the vapor pressure of water from the atmospheric pressure, or whether to calculate the molecular weight without any correction. If the air in the vaporization tube is saturated at room temperature, in very humid weather It is easy to approach this condition, so of course there is no need to calibrate at this time; on the contrary, if the air is very dry, and if the water in the measuring tube has enough time to saturate the air in the tube, all calibrations should be made. If you measure the relative humidity, and assume that there is enough time to The exhausted air is saturated, and you can roughly judge the size of the correction value.

Please explain how to perform accuracy analysis based on this experiment. Please review the discussion section and make appropriate changes to the device you want to use. .In your error score, the uncertainty affected by water vapor pressure does not need to be considered. Calculate with both full correction and no correction, and then determine which one is the better value.

Please see About Discussion of barometers to determine whether appropriate corrections are needed for atmospheric pressure readings.

Instructions to instructor

A series of mixtures of ether and chloroform can be used as unknowns. The values ??of these mixtures For detailed preparation, see H.D, Crockford, J.Chem.Ed. , 27, 126 (1950)

In addition, low-boiling Freon, methylene chloride, or other chlorinated- or fluorinated-hydrocarbons are also suitable for this experiment.