Determination Of The Molar Mass Of A Metal
In chemistry, we are often faced with the task of identifying an unknown substance. Scientists may face this challenge when identifying ingredients in a rock formation, an environmental testing site, or even a planet or star. Depending on the situation, different methods can be used to identify the material. In this experiment, you will be given a sample of an unknown metal. By performing a chemical change on the metal and conducting some measurements, you can calculate the molar mass of the metal. After comparing this molar mass to those on the periodic table, you will be able to identify the mystery metal.
In order to determine the molar mass of the metal, we will react a sample of the metal with hydrochloric acid. This reaction produces hydrogen gas and an ionic compound that contains metal and chloride ions. The mass of this metal chloride product will be measured. Using our data, we can calculate the grams of metal and moles of metal used in the experiment, which we will use to calculate the molar mass of the metal.
The metal you use in this experiment will be one that loses 2 electrons when forming ions. This information allows you to determine the ratio of ions in the compound and to write its formula. (If we use the symbol M for the mystery metal, what would the formula be for an ionic compound containing metal and chloride ions?)
Procedure:
1. Obtain a clean, dry evaporating dish and a watch glass. The watch glass will be used as a cover.
2. Weigh the empty dish and cover and record the mass on the report sheet to three decimal places (three digits after the decimal point).
3. Add between 0.250 and 0.300 g of metal to the evaporating dish. Weigh the dish, cover, and metal, and record the mass to three decimal places.
4. Take the dish to a fume hood. Under the hood, add 5.0 mL of dilute hydrochloric acid (HCl) to the metal in the evaporating dish. Place the watch glass cover over the dish. Allow all of the metal to react; when the reaction is complete, no solid should be present in the dish.
5. When all the metal has reacted, place the covered evaporating dish on a hot plate and evaporate to dryness. The liquid should be boiling (bubbling). Do not melt the solid residue, but be sure all the vapors of water and acid have ceased to be evolved. Heat the dish until no moisture can be seen in the dish or on the watch glass cover. The metal chloride residue should appear porcelain white.
6. Allow the dish to cool to room temperature, then weigh the dish, glass, and metal chloride. Record the mass to three decimal places.
7. Repeat the entire procedure with another sample of metal.
8. Using your data from lines 1, 2, and 3 on the report sheet, calculate the masses for lines 4, 5, and 6 to three decimal places.
9. Calculate the moles of metal used (line 7). Do not round this answer—record all digits from your calculator!
10. Calculate the molar mass of the metal (line 8) to one decimal place.
11. Calculate the average molar mass of the metal (line 9) to one decimal place. Using this average and the periodic table, identify your metal on the report sheet and record its molar mass as listed on the periodic table.
12. Calculate the % error in your answer using the formula below:
|average molar mass – accepted molar mass| / accepted molar mass x 100
Note: Show all calculations on the report sheet!
#4: Mass of metal chloride: Trial #1: 0.983g Trial #2: 0.979g
#5: Mass of metal:
Trial #1: 0.266g Trial #2: 0.251g
Steps for calculations:
#6: mass of chloride (Cl2)
You know the mass of metal chloride and metal
Mass of metal chloride – mass of metal = ________________g Cl2
#7 Moles of metal used:
________g Cl2 x 1 mole Cl2/71g Cl2 = _________ mol Cl2
________ mol Cl2 x 1 mol M/1 mol Cl2 = ________ mol M
#8 Calculated molar mass of metal:
gM/_________molesM = __________g/mol M
*the metal “M” will be in Group #2 of the Periodic Table