Molar Volume Lab

Molar Volume Lab Problem: Using the following reaction, determine the molar volume of hydrogen gas produced. Mg + 2HCl  MgCl2 + H2 Procedure:  Begin with a few centimeters of magnesium ribbon ( you must determine its mass).  Obtain a collecting tube (eudiometer) from you instructor, who will add approximately 5.0 ml of HCl (12M).  Fill the tube with water.  Roll the magnesium into a small coil, secured with string, and push the coil into the tube with the thread extending.  Invert the tube in a large beaker about half-filled with water. As the higher density HCl falls to the bottom of the tube the reaction will proceed.  When the reaction has stopped, raise your tube out of the water until the height of the water inside the tube is the same as the height of the water in the bucket. Record the volume of gas inside the tube. Remember to read the volume at the bottom of the meniscus. Be aware that other gases, notably water vapor, will contaminate your hydrogen gas and you will need to account for the partial pressure of the water vapor in your product. Information about the partial pressure of water vapor can be found in your textbook.  Record the temperature and barometric pressure in the room.  CLEAN UP.

Analysis:  Determine an accurate measurement of the volume of hydrogen produced and convert to STP using the following steps: 1. Correct the barometric pressure for the presence of water vapor 2. Convert the volume to STP  Determine the number of moles of magnesium used in the reaction and use the stoichiometry of the reaction to determine the number of moles of hydrogen gas produced.

 Use your liters of gas and moles of gas produced to find the volume of one mole of hydrogen gas at STP.

 Calculate your percent error, compared to the standard molar volume of a gas.

Mass of Mg g

Volume of gas produced (V1) ml Barometric pressure mmHg Temperature °C PRE-LAB QUESTIONS 1. When the tube is inverted, how does the magnesium react with the acid if the magnesium is at the bottom of the tube and the acid is at the top? Explains what happens and why. 2. How do you know that the reaction has stopped? 3. When reading the gas volume in the tube, what is the purpose of equalizing the water level in the bucket with the water level in the tube? 4. Write the balanced chemical reaction that occurs. 5. Calculate the mass of magnesium metal needed to collect exactly 80 mL of H2 gas from the reaction above. Assume standard conditions.

POST-LAB QUESTIONS 1. Which assumption made in your calculations produced the biggest source of error?

2. Although the chemical reaction in this experiment produces only H2 gas, there was another gas present in the tube. What was it? 3. Why was this gas there?