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1. To understand a method for determining the molar gas constant.
2. To be familiar with the application of partial pressure law and gas equation of state.
3. To practice using of analytical balances and operating of measuring gas volumes.
The expression of the gas state equation is:
In the formula:p——gas pressure or partial pressure(Pa)
V——Gas volume(L)
n——the amount of gas substance(mol)
m——mass of gas(g)
M r ——The molar mass of the gas(g·mol -1 )
T——gas temperature(K)
R——mole gas constant(document value:8.31 Pa·m 3 ·K -1 ·mol -1 or J·K -1 ·mol -1 )
Therefore,for a certain amount of gas,if the volume occupied be measured under a certain temperature and pressure conditions,the gas constant can be obtained.In this experiment,aluminum is used to react with hydrochloric acid.
The generated hydrogen is approximately considered to be an ideal gas under experimental conditions,and then the volume is collected and measured by the drainage gas collection method,thereby the gas constant could be determined.
Since hydrogen is collected above the water surface,the partial pressure of hydrogen[p(H 2 )]is related to the saturated vapor pressure of water[p(H 2 O)].According to the law of partial pressure:p=p(H 2 )+p(H 2 O),then p(H 2 )=p-p(H 2 O)while p is atmospheric pressure and can be read by a barometer.
The amount of hydrogen can be calculated from the reaction formula based on the weight and atomic weight of aluminum.
Since p(H 2 ),V(H 2 ),n(H 2 )and T can be measured experimentally,the gas constant can be obtained according to the gas state equation.
Analytical balance;barometer;precision thermometer;graduated cylinder(10mL);device for generating and measuring hydrogen volume;aluminum sheet.
6 mol·L -1 HCl.
Take a small piece of aluminum and weigh it on an electronic balance,and its weight must be between(20~30)mg(do not overweight the aluminum piece to avoid the volume of hydrogen generated exceeding the measurement limit of the gas measuring tube),and record the weight of the aluminum piece in Table 3-1.
Figure 3-1 Gas constant measuring device
The experimental setup is shown in figure 3-1.Add the suitable water to the level bottle.Move the level bottle up and down slightly so that the two liquid levels of the level bottle and the measuring tube are on the same level,and should be near the 0 scale.Tighten the rubber stopper on the test tube.Move the level bottle down to about 30 marks on the gas measuring tube,and the liquid level in the gas measuring tube will accordingly drops,but it will be stable after a short period of decline.Continue observing for a few minutes to confirm that the liquid level no longer drops,which indicate that the experimental device does not leak air,and then the following operations should be continue.(If the liquid level continues to drop,even down to the bottom of the level bottle,it means that the experimental device is leaking.You must find out the problem and fix it before the operation below).
Remove the rubber stopper and use a long-necked funnel to add 5mL of 6 mol·L -1 HCl to the reaction tube to avoid HCl sticking to the wall of the test tube.Dip the aluminum flakes in a small amount of water and use a glass rod to send it along the wall into the test tube.(Note:the aluminum flakes cannot be in contact with hydrochloric acid.)Make it stick to the wall of the test tube.Record the data in Table 3-1.
Connect the test tube to the rubber stopper tightly,adjust the height of the funnel to the same level as the liquid level of the gas measuring tube,and stabilize it near the 0 scale(if necessary,check the experimental device again for leaks),record the the gas pipe liquid level scale reads V 1 .Gently shake the test tube(but do not remove it)to make the aluminum piece fall into hydrochloric acid.The liquid level of the gas pipe will drop continuously due to the pressure generated by the reaction between aluminum and hydrochloric acid.(Note:As the reaction happens,the funnel should be slowly moved down accordingly so that the liquid level in the gas measuring tube and the funnel are basically on the same plane to prevent the pressure in the gas measuring tube from being too high and causing gas leak).After the reaction is stopped,the test tube is cooled down to room temperature,and the two liquid levels are adjusted to be on the same horizontal surface,and then read the liquid level calibration as V 2 .Record the data in Table 3-1.Measure and record the experimental temperature T and atmospheric pressure p.Record the data in Table 3-1.
Table 3-1 Determination of molar gas constant
1. Why is it necessary to check the experimental device for leaks?During the experiment,the device should be checked twice for leaks.Which one is more important?
2. When reading the liquid level of a gas measuring tube,why should the two liquid levels of the funnel and the gas measuring tube be on the same level?
