Collecting Gas Over Water

Purpose:

(put in lab book)

The purpose of this lab is to determine the mass of a magnesium reacted with hydrochloric acid by collecting the gas produced.

Procedure:

(Read, then draw pictures describing the setup. You can include small descriptions with your pictures like a comic strip.)

Initial Preparations and Data

Look up Franklinville on weather.com and find the pressure today. The pressure will have the unit in. This is really in Hg (inches of Mercury). The atmospheric pressure is the pressure in the room which will also be the pressure of the gas we collect.

Fill a 1000 mL beaker almost full with tap water. Let it sit while you prepare the rest of the experiment. We want the water to be close to room temperature. We will need to take the temperature of the water after completing the reaction.

Obtain a strip of magnesium metal and measure the length.

Assembly of Apparatus

Roll the magnesium strip into a coil, tie it to a string, and them wrap the string around the coil. Show it to me so that I can approve it, then set this assembly aside for use later in the experiment.

Obtain a 50 mL gas collection tube (eudiometer), which is a graduated tube open at one end and closed at the other. COMPLETE THIS STEP IN THE HOOD USING GLOVES - Using a 10 mL graduated cylinder, carefully add 8-10 mL of concentrated hydrochloric acid into the eudiometer. Hold the eudiometer at a 45° angle and very carefully and slowly fill the eudiometer with water (use some of the water from the 1000 mL beaker). You are slowing adding it so that it will not mix with the acid. The eudiometer should be filled to the point that the water is almost overflowing.

Insert the string holding the magnesium into the open end of the eudiometer. Lower it a few centimeters into the water. Leave some of the string hanging out to be held by the stopper and seal the eudiometer with the stopper. (Push the stopper straight into the eudiometer until it cannot be pushed any further and no more water bubbles out of the eudiometer when pushing on the stopper. No air should be in the eudiometer.)

Reaction of Mg and HCl; Collection of Hydrogen Gas

Keeping your finger tightly over the hole in the stopper, invert the eudiometer into the 1000 mL beaker on your laboratory table. Remove your finger from the hole in the stopper and clamp the eudiometer to a buret clamp supported by a ring stand making sure that the stoppered end of the eudiometer remains submerged in the water in the beaker.

At this point you should be able to see the acid layer swirling down through the water layer toward the magnesium wrapped in the string. The reaction is complete when no more bubbling is seen around the magnesium metal. At this point, lightly tap the eudiometer to dislodge any bubbles that remain around the metal.

In your data table, record the volume reading on the gas collection tube. Using a thermometer, measure the temperature of the water in the beaker and record it accurately in your data table.

Data:

(make a table or list in lab book of all data collected)

Calculations:

(complete in lab book)

1. Write a balanced equation for the reaction that took place.

2. Calculate the moles of hydrogen gas produced. (Remember you collected it over water).

3. Calculate the observed mass of magnesium based on the amount of hydrogen you produced.

4. Calculate the true mass of magnesium. (1 meter of magnesium has a mass of 1.80g)

5. Determine your percent error.

Questions:

(answer in lab book)

1. What happens to the acid after the eudiometer is inverted and why?

2. Why does there need to be a hole in the stopper?

3. In your reaction, the magnesium is the limiting reactant, so that it is completely consumed in the reaction. The amount of hydrochloric acid added to the reaction was more than enough to consume all of the magnesium. Calculate the exact volume of 12 M hydrochloric acid necessary to consume your mass of magnesium metal.

4. Identify an error in this lab and how you would fix it.