MOLAR MASS OF MAGNESIUM
I. Purpose –Determine the molar mass of magnesium using simple lab equipment.
II.Procedure –
a.Obtain a piece of magnesium (Mg) ribbon less than 4 cm long. Measure the length of the ribbon carefully and record this to the nearest 0.01 cm. Your instructor will give you the mass of 100 cm of the ribbon. Since it is uniform in thickness, you can calculate the mass of the magnesium used.
b.Fold the magnesium ribbon so that it can be encased in small spiral case made of fine copper wire. Let enough copper wire serve as a handle so that the cage can be put about 5 cm down from the end of the tube. See Figure 1.
c.Set up a ring stand and buret clamp in position to hold a 50 mL gas measuring tube which has been fitted with a one- or two-hole rubber stopper, as shown in Figure 2. Place a wash pan about two-thirds full of tap water near the ring stand.
d.Tilt the gas measuring tube slightly and pour in 6 M hydrochloric acid to about the 10 mL mark.
e.With the tube still tilted, slowly fill it with tap water from a beaker. While pouring, rinse any acid that may be on the sides of the tube so that the final liquid in the top of the tube will contain very little acid. Try to avoid stirring up the acid layer in the bottom of the tube.
f.Holding the copper coil by the handle, insert it into the tube until the cage is positioned about 5 cm down from the open end. Hook the copper wire over the edge of the tube and clamp it there by inserting the rubber stopper. When properly set up, the tube will contain no air bubbles and the water will completely fill the hole(s) in the stopper as well as the tube. See Figure 2.
Figure 1Figure 2
g.Cover the holes in the stopper with your finger and invert the tube in the container of water. Clamp the tube in place. The acid, being more dense than water, will stream down through the water and will eventually react with the metal.
h.After the reaction stops, wait about 5 minutes to allow the tube and its contents to come to room temperature. Bubbles clinging to the sides of the tube can be dislodged by tapping the tube gently.
i.Cover the hole(s) in the stopper with your finger and transfer the tube to a large cylinder or battery jar which is almost filled with water at room temperature. Raise or lower until the level of the liquid inside the tube is the same as the level outside the tube. This permits you to measure the volume of the gases in the tube (hydrogen and water vapor) at the air pressure of the room. Read the volume with your eye at the same level as the bottom of the meniscus (the lens-shaped surface formed by the water in the tube). Record the volume of the gas to the nearest 0.01 mL.
j.Remove the gas measuring tube from the water and pour the acid solution it contains down the sink. Rinse the tube with tap water.
k.Repeat the experiment two more times using different lengths of Mg ribbon.
l.Record the room temperature. Use this temperature to select the proper vapor pressure for water.
III.Formal Report – Answer the following questions in your formal report. If the answer requires a calculation, show your work.
1.Calculate the number of moles of hydrogen gas produced assuming ideal gas behavior. Remember to correct for the vapor pressure of water.
2.Write a balanced equation for the reaction of magnesium with hydrochloric acid.
3.Write the molar ratio: from the balanced equation.
4.Calculate moles of magnesium used up in the reaction:
5.Calculate molar mass of magnesium =
MOLAR MASS OF MAGNESIUM DATA SHEET
Name______Date______Lab Section __
Data Trial 1 Trial 2 Trial 3
1. Length of magnesium ribbon ( 0.01 cm) ______
2. Mass/length ratio of magnesium ______
3. Mass of magnesium ______
4. Barometric pressure ( 0.1 mm Hg) ______
5. Room Temperature ( 0.1C) ______
6. Vapor pressure of water (from CRC Handbook ______
of Chemistry & Physics) (units of mm Hg)
7. Reading of gas measuring tube ( 0.01 mL) ______
(volume of hydrogen gas)
Formal Report - Answer the following questions in your formal report. If the answer requires a calculation, show your work.
1. Moles of hydrogen gas ______
2. Balanced chemical equation: ______
3. Molar ratio = moles magnesium/ moles hydrogen gas
4. Moles of magnesium ______
5. Molar mass of magnesium ______
6. Average value of molar mass ______