Purpose: to determine the experimental empirical formula of a compound, magnesium oxide, and compare it to its theoretical empirical formula, MgO.
Materials: (list the materials you have on your table)
- Clean a crucible and lid, rinsing thoroughly with deionized water as a last step. (It will not be possible to get a used crucible completely clean.) Dry the crucible and lid with a paper towel. Check the crucible for cracks. Your instructor will tell you how.
- Place the clean, dry crucible and lid on a clay triangle on a ring stand and heat strongly with a Bunsen burner (blue cone) for 5 minutes to remove any volatile material. Use crucible tongs to carefully move the hot crucible and lid to a tile on the counter to cool completely. USE CAUTION TO AVOID BURNS! Hot ceramic and hot metal look just like cold ceramic and cold metal. Once the crucible and lid are completely cool (room temperature), weigh them on the milligram balance and record the exact mass on the data table.
- Obtain a piece of Magnesium ribbon that weighs about 0.3 g. (Do not take time to measure a piece of exactly 0.300 g, just get close and record the exact mass used in the data table.) Crumple the ribbon and pack it into the bottom of the preheated and pre-weighed crucible. Weigh the crucible, crucible lid, and magnesium on the digital balance and record the exact mass on the data table.
- Place the crucible in the triangle as illustrated, with the crucible lid tilted slightly to allow air into the crucible. Heat strongly with a Bunsen burner (blue cone). NEVER LOOK DIRECTLY AT BURNING MAGNESIUM. If the magnesium starts to burn (very bright light emitted) and “smoke” is given off, use crucible tongs to cover the crucible completely with the crucible lid for a short period of time. After several minutes, tilt the crucible lid open again. Repeat the same procedure until no more “smoke” is observed and the contents of the crucible no longer glow brightly as the crucible is heated. Heat the crucible for several more minutes with the lid off completely so that the bottom of the crucible glows a dull red. Remove heat and allow the crucible to cool completely on the clay triangle.
- When the crucible has cooled, add 6 to 8 drops of distilled water to the contents of the crucible, using a medicine dropper. Water is added to react with and then remove any nitrogen that may have been added to your sample from the air during heating. Heat the crucible gently with the cover off for 5 more minutes. Remove from heat and allow the crucible to cool. Use crucible tongs to move the crucible and lid to a tile on the counter to cool completely. Weigh the crucible, crucible lid, and product on the milligram balance, and record the exact mass on the data table.
Calculations: SHOW YOUR WORK
a) Based on the data in your Table, what mass of magnesium is contained in your compound? Show your calculation.
b) Based on the data in your Table, what mass of oxygen is contained in your compound? Show your calculation.
c) Compare the mass of the Mg ribbon with the mass of the magnesium oxide (rows 2 and 3 in your Data Table). Notice that the mass of the magnesium oxide is greater than the mass of the Mg. How do you account for this apparent increase in mass?
d) Now that you have the mass of magnesium and oxygen in your compound, you can find moles of each element in the compound and you can determine your experimental empirical formula. Show your calculations and your empirical formula below.
Analysis Questions: Show your work!
- The percent by mass composition of a salt was found to be 56.58% potassium, 8.68% carbon, and 34.73% oxygen. What is the empirical formula of this salt?
- A compound containing iron and sulfur was formed by combining 2.233 g of iron with 1.926 g of sulfur. What is the empirical formula of the compound?
- Propylene has a molar mass of 42.00 g/mole and is composed of 14.3 % hydrogen and 85.7 % carbon. What is the molecular formula of propylene?
- A sample of an 10 g of oxide of nitrogen is found to contain 3.04g nitrogen. What is its empirical formula?
- A sample of an oxide of arsenic is found to contain 75.74% arsenic. What is its empirical formula?
- What is the empirical formula for a compound containing 26.57g potassium, 35.36g chromium, and 38.07g oxygen?
- What is the empirical formula of a compound comprised of 1.8% hydrogen, 56.1% sulfur and 42.1% oxygen?
- A borane is a compound containing only boron and hydrogen. If a borane is found to contain 88.45% boron, what is its empirical formula?
- Find the empirical formula for a compound containing 40.6% carbon, 5.1% hydrogen, and 54.2% oxygen.
- What is the empirical formula of a compound containing 47.37% carbon, 10.59% hydrogen and 42.04% oxygen?