Experimental Procedure and Data

1. Have your instructor check your set up before you proceed with the lab. It is important to have your set-up ready before you move on.
2. Using a graduated cylinder, transfer 5.0 mL of the standard 4M HCl into the flask.
3. You will be supplied with a sample of either of three metals, Ca, Al or Mg. Tare a weigh boat on an analytical balance. Accurately weight ~0.04-0.05 g of the metal in the weigh boat.
4. Record the exact mass to be used in your data section (you should have three decimal points).
5. Pour the entire metal sample from the weigh boat directly into the flask containing the HCl solution and immediately stopper the flask. Be careful that all of the metal gets into the acid and does not stick to the side of the flask. If it does, gently swirl the flask to release the metal into the acid.
6. The hydrogen gas produced by the reaction will displace water from the measuring cylinder. Calcium and magnesium will react vigorously; aluminium will react rapidly after a short induction period of several minutes.
7. If you do NOT see the bubbles being released into the graduated cylinder, you may need to firmly hold the stopper onto the flask to make sure there is not a leak.
8. When reaction is complete (no further evolution of gas) cool the flask by holding it in the water bath for a few minutes if it has become too hot. When the flask is a room temperature, raise or lower the measuring cylinder to equalise the water levels inside and out, as illustrated below, and record the volume of gas evolved.
9. Once the water level is equal to the level of gas in the cylinder, record the volume of gas produced in your data section.
10. Run two trials. Record all data.

Pre-lab Assignment/Questions

*Note– this pre-lab must be finished before you come to lab.

1. Use Equation 1 to write the formula for magnesium chloride and balance the following equation for the reaction of magnesium with hydrochloric acid.

Mg+_________HCl →MgCl_____+ _______H_2(g)

2. If the metal used was Al, use Equation 1 to determine the mol ration of metal to hydrogen gas.

______ mol Al = _____moles of H₂

3. Determine the percent composition of the metal in the AlCl3.

4. A student performs the lab on an unknown metal and obtains the following data:

Using this data, complete the following calculations:

1. moles of H₂ gas = (V * P) / (R * T): ________________
2. Molar ratio of H:M assuming a 1:1 ratio of M to H2. We will assume a 1:1 molar ratio for this experiment.
3. Moles of metal used: ________________
4. Atomic mass of metal = g metal used / moles of metal. ________________

Possible identity of the metal: ____________________

Experimental Data and Results

Metal Used __________________________________________

Calculations

Average atomic mass of metal ___________________

Atomic mass of metal of from the periodic table _____________

% Error of your measured atomic mass of the metal ____________________

Results, Discussions and Post-Lab Questions

1. Determine the percent composition of the metal in the metal chloride produced in your experiment.

2. The measurement of the mass of metal should be precise to 0.001 g in 0.5 g; that is, to 0.2%. Thus the largest source of error in obtaining the atomic mass of the metal is likely to be the measurement of the volume of gas evolved.

a. Calculate the atomic mass of the metal if the volume measured in the prelab problem #4 was 30.0 mL instead of 26.5 mL still assuming a 1 to 1 ratio between the moles of metal used and the moles of H₂ gas produced.

b. What then would be the best estimate of the metal used?

3. Copper is a transition metal that can have more than one charge. Write the equation of Cu⁺ reacting with hydrochloric acid. Then write the reaction of Cu²⁺ reacting with hydrochloric acid. How does the amount of hydrogen gas evolved change with each?

4. Give two possible sources of error in your experiment. Explain how would each affect the true value (would the atomic mass of the metal be calculated too high, too low or be unaffected)?