Predicting Volumes Using Moles Name:

Intro Today you will be torturing a small piece of magnesium metal, by tying it up in copper wire and submitting it to an acid bath. Given the following reaction, you should be able to calculate the volume of hydrogen produced by utilizing the ideal gas equation.

Procedure 1) Obtain a piece of magnesium (Mg) ribbon 8) Continue to fill the tube until it and measure its length as precisely as will hold no more water. possible. Dislodge bubbles by tapping. 2) Crumple (do not tightly fold) the 9) Without dropping the magnesium ribbon so that it can be magnesium ribbon, place it encased in a cage made of copper wire. inside the tube so that it extends The wire is also at the lab bench. about 3 cm into the tube and 3) Make sure your copper wire cage encircles hook the remaining wire over the magnesium, and has about 5 cm of the side of the tube. copper left as a handle. 10) Place a stopper on the end of the tube, put your finger Mg ribbon Help! over the hole, and place Save me! the tube, stopper end first into the large beaker of water. Clamp the tube so that the stoppered end is 5 cm below the level of the water. 4) Set up a ring stand with a test tube holder. 11) The acid which is more And place a large beaker filled 3/4 full dense than the water will with water, beneath test tube clamp. This flow down the tube and will be used later. react with the magnesium. 5) Obtain a eudiometer (long glass tube). 12) After the reaction stops, Caution 6) : strong acid. Tilting the tube to wait about five minute to allow the tube the side, carefully pour about 10 ml of 6M to come to room temperature, and HCl into the tube and keep the tube in dislodge any bubbles clinging to the side this position. (You should pour the acid by tapping the tube. into a small beaker first.) 13) Lower the tube into your beaker until 7) Now fill the rest of the tube with tap the water level inside the tube is equal to water from a clean small beaker. This the water level in the beaker. If you must be done extremely carefully, cannot lower the beaker far enough, just because you are trying not to disturb the lower it as far as possible. acid layer at the bottom of the tube. 14) Record the volume of gas produced, the While pouring try to rinse any acid that temperature of the gas (room temp.), and may be on the side down to the bottom of the pressure from the barometer. the tube. Calculations (Neatly done on a separate piece of paper)

1) Get from the board the number of grams per centimeter for the magnesium ribbon. Calculate the mass of the Mg ribbon you used.

2) How many moles of magnesium did you use?

3) The reaction was as follows: Mg(s) + 2 HCl(aq) ---> H2(g) + MgCl2(aq) Using the ideal gas equation calculate the volume of gas that should have been produced.

Given the volume you actually produced, what was your percent error?

4) The volume calculated above will not be completely accurate because some of the water has evaporated into the space in the tube. These molecules exert a pressure called the vapor pressure. Look up the vapor pressure of water at our room temp. (Record the closest possible value from the table on p. 446).

Use the ideal gas equation to predict how many moles of water vapor are in the eudiometer. (P = vapor pressure)

5) Re-calculate what volume should be inside the eudiometer, assuming that the moles of gas (n) will be equal to the moles of hydrogen produced plus the moles of water vapor.

6) What was your true percent error?