R-Lab

The purpose of this laboratory experiment is to measure the universal gas constant R in the Ideal Gas Law. This can be accomplished by reacting hydrochloric acid and magnesium, trapping the hydrogen gas that is produced by the reaction, and measuring its pressure, volume and temperature. The number of moles of hydrogen gas produced from the original mass of magnesium can be determined as well, using stoichiometry and the equation below,.

2HCl (aq) + Mg (s)	→	MgCl2 (aq) + H2 (g)

The ideal gas law and the quantities measured during the lab (P,V,T & n) will be used to calculate R.

SAFETY – SAFETY GLASSES MUST BE WORN AT ALL TIMES. Be careful when working with acids, they are corrosive and if spills occur, please rinse immediately. Spilling on clothes may cause damage. Aprons are available in the back of the classroom if desired. Be careful!

Procedure:

Using an electric balance, record the mass of magnesium ribbon provided.

Wrap the copper wire around the magnesium ribbon, making a cage that surrounds the ribbon as demonstrated by the instructor. Leave a handle of copper wire approximately 6 cm long.

Insert the handle end of the copper wire into the bottom of a one-hole rubber stopper.

Fill the large beaker with water.

See the instructor to obtain approximately 5.0 mL of HCl.

Slowly with a wash bottle gently fill the eudiometer by drizzling water down the eudiometer’s inner side to avoid mixing. The HCl should stay on the bottom and the water should be filled completely, all the way up to the very top.

Insert the stopper w/copper cage into the eudiometer by tapping gently as to avoid cracking the eudiometer. The copper wire cage should be suspended at the top of the eudiometer. While holding a finger over the hole in the rubber stopper; quickly invert the eudiometer into the beaker of water. When the top of the eudiometer is underwater, the finger can be removed.

Carefully secure the eudiometer in the buret clamp, being sure to keep the bottom underneath the surface of the water in the beaker at all times.

Record all observations.

When the Mg ribbon is no longer reacting gently tap the side of the eudiometer to release any trapped bubbles. Using the thermometer, read and record the temperature of the water in the beaker.

Record the atmospheric pressure as indicated by the instructor.

Determine the water vapor pressure from the table provided by the instructor.

Read and record the volume of gas in the eudiometer. Remember to measure how much gas was produced –	a eudiometer totally full with liquid would contain 0.00mL of gas.

Remove the eudiometer from the beaker and dispose of the contents of the beaker by pouring it down the drain.

Remove the stopper and note the appearance of the copper wire. Use the wash bottle to carefully rinse the eudiometer with 3-5 mL of water, twice.

Straighten out the copper wire for the next group, put everything away and clean up the work area.

Pre-Lab Questions:

Solve the ideal gas equation for R in terms of all the other variables.

What will be the volume of 0.21 moles of air at Standard Temperature and Pressure (STP = 1atm and 273K)

Start Lab:

Data: (Draw data table below)

Calculations:

Write a balanced chemical equation for the reaction that takes place between magnesium and hydrochloric acid.

What type of reaction is this?

Convert the mass of magnesium ribbon to moles of magnesium. Use stoichiometry to find the moles of H2 gas produced.

Use the temperature measurement and the water vapor pressure table copied in class to correct for the partial pressure of water in the gas trapped in the eudiometer. What is the actual pressure of the hydrogen gas produced?

PH2 = Patmosphere – PH2O

Convert the volume of gas produced from mL to liters.

Convert the temperature of the water bath to Kelvin.

Use the classroom data, NOT STP CONDITIONS, to calculate R for the H2 produced. Use the ideal gas law to calculate the experimental ideal gas law constant, R.

Compare calculated value of R with the accepted value of R in the textbook. And calculate a percent error.

% Error = │Actual Value – Expected Value │ x 100

Expected Value

Post-Lab Questions:

Discuss your errors with your group. What could you do differently in the future to compensate for those errors, if you were given an opportunity to do this lab again?

In setting up this experiment, a student noticed that a bubble leaked into the eudiometer when it was inverted in the water bath. What effect would this have on the measured volume of gas? Would the calculated molar volume of hydrogen be too high or too low as a result of the error? Explain.