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Empirical Formula of a Compound

Abstract

The following experiment will open one’s insight on empirical formula of compound. The main aim purpose of this experiment will be to determine the empirical formula of a given compound experimentally. However, due to errors made in the experiment and human errors, the final formula obtained may not be the actual formula for the compound. We shall however try a much as possible to avoid various errors so as to increase the accuracy of the results obtained.

Precautions

Before commencing the experiment, a number of safety measures ought to be observed. Magnesium ribbon is flammable and as such should be properly handled. Nitric acid is very toxic and highly corrosive. It should be handled carefully so as not to come into contact with neither skin nor eyes. Ammonia is also a very toxic and harmful gas. Any hot apparatus should be handled properly so as not to cause burns.

 

Introduction

Empirical formula of a compound refers to the simplest ratio of moles of the atoms of a compound. The ratio of these atoms are indicated in the chemical formula by subscript numbers. It is important to know the difference between the empirical formula and chemical formula of compounds. The empirical formula may be chemical formula of a compound. However, the chemical formula may not necessarily be the empirical formula. For example. The chemical formula for glucose is C6H12O6, while its empirical formula is CH2O. On the other hand, a compound such as carbon (IV) oxide has its chemical and empirical formula as CO2.

The empirical formula of a compound may be determined experimentally if the chemical is synthesized directly from its constituent elements. There are three simple steps to be followed:

  1. The mass of each element is determined
  2. Calculate the number of moles of each element in the given sample
  3. The molar ratio of the elements is then expressed as whole numbers.

Let us take aluminum oxide as an example. A solid sample was found to have 1.7grams of aluminum and 1.57grams of oxygen. The simplest ratio obtained is 2:3 as shown below:

Molar mass of aluminum is 26.98g Al/mol Al

1.76g Al/ 26.98g Al/mol Al = 0.0652 moles of Al

Molar mass of oxygen is 16.00g O/mol O

1.57g O/ 16.00g O/mol O = 0.0981 moles of O

To obtain the mole ratio, divide each of the number of moles by the smallest number, which is 0.0652.

Moles ratio of Al = 0.0652/0.0652 = 1.00

Mole ratio of O = 0.0981/0.0652 = 1.50

1.00: 1.50 in the simplest whole number is 2:3

This means that for every two atoms of aluminum there are three atoms of oxygen. Molecular oxygen is very reactive whether it is a pure element or found in a mixture such as air. Nitrogen the major component of air is typically unreactive. When an element reacts with oxygen in a process known as combustion, an oxide is formed. In air, nitrogen is also available, and as such, some nitrides may be formed. In this experiment, a piece of magnesium ribbon and air will be reacted to form magnesium oxide. The empirical formula of the magnesium oxide will be determined using the initial mass of metal and the final mass of the metal oxide.

  • Materials and Reagents
  • Magnesium ribbon
  • Crucible
  • 6M Nitric acid
  • Sandpaper or steel wool
  • Heat

 

Procedure

  1. Obtain a crucible and lid. Clean the crucible and check for any stress cracks, fractures, or fissures. These types of defects are common in used crucibles. If the crucible is dirty then move the apparatus to a hood and add 1-2 mL of 6M HNO3 and gently evaporate to dryness then inspect the crucible after cooling for any defects. If no defects are found the crucible and lid should be supported on a clay triangle.
  2. Heat the crucible with a gentle flame for 5 minutes before heating with an intense flame. Continue heating for 10 to 12 minutes with an intense flame after the bottom of the crucible has become red. Allow the crucible to cool by placing it on a wire pad. Do not set the crucible on the bench top or touch it with your hands due to possible contamination or cracking.
  3. Determine the mass of the “fired” cool crucible with lid and record.
  4. Repeat step 2 and 3 until you have two crucible and lid mass readings that differ by no more than 0.010 g.
  5. Obtain a magnesium ribbon that weighs between 0.17 and 0.23 grams. Form the ribbon into a loose ball or coil so it rests in the bottom of the crucible. If the ribbon is not bright and shiny, clean the surface with a piece of sandpaper or steel wool to remove any impurities. DO NOT SAND DIRECTLY ON THE DESKTOP!
  6. Add the clean coiled magnesium ribbon to your crucible. Weigh the crucible containing the metal with the lid.
  7. Return the crucible and lid containing the metal to the clay triangle support. Initially heat the sample gently for 2 to 3 minutes, gradually intensify the heat, and continue heating for three minutes.
  8. Using the crucible tongs, slightly lift one edge of the lid to allow air inside of the crucible. DO NOT REMOVE THE LID. When done correctly, the metal will begin to glow. If the metal ignites into a flame, quickly cover the crucible with the lid. Heat the crucible for approximately 3 minutes.
  9. Repeat step 8 several times until no glowing metal is seen or remains upon the entrance of the air. Then remove from the heat source, cover with the lid, and cool to room temperature.
  10. To the cooled crucible with no remaining magnesium metal, add 3 drops of distilled water. The smell of ammonia may be evident.
  11. Position the crucible lid slightly off to the side to allow the evolving water molecules to escape during heating. Initially heat the sample slowly and gradually intensify the heat. Be careful not to let the crucible become very hot too fast or liquid will splatter out of the crucible. Heat the sample at a high temperature for 15 to 17 minutes.
  12. Allow the crucible and the metal oxide to cool. Determine the mass of the crucible, lid, and metal oxide using the same balance as used in the earlier steps.
  13. Reheat the sample for an additional 5 minutes with high heat. Measure the combined mass of the crucible, lid, and metal oxide; repeat this process until you obtain 2 concurrent readings within 0.010 g of each other.
  14. Repeat this entire procedure with a new sample of magnesium metal for Trial 2.

 

 

Results and Calculations

 

  First trial Second Trial
Mass of magnesium ribbon 0.2100 0.2100
Mass of crucible and lid after first heating 25.5328 20.0748
Mass of crucible and lid after second heating 25.6928 20.0688
Mass of crucible lid and metal 0.1018 0.1598
Mass of crucible lid and metal after first heating    
Mass of crucible lid and metal after second heating 25.6928 20.2528

 

Mass of metal oxide

First trial: 25.6928-25.5338= 0.1598g

Molar mass of MgO = 24.315

Number of moles = 0.1598/ 24.315 = 0.0065

 

Mass of oxygen

Trial one: 0.1598-0.1018 = 0.057g

Molar mass of oxygen = 16.00g

Number of moles = 0.057/16.00 = 0.0036

Mole ratio = (0.0063/0.0036): (0.0036/0.0036) = 1.75:1

From this, the ratio of atoms is approximately 7:4

 

Second trial: 20.2528-20.0688= 0.1848g

Molar mass of MgO = 24.315

Number of moles = 0.1848/ 24.315 = 0.0076

 

 

Trial two: 0.1898- 0.1068 = 0.083g

Molar mass of oxygen = 16.00g

Number of moles = 0.036/16.00 = 0.0048

Mole ratio = (0.0076/0.0048): (0.0048/0.0048) = 1.58:1

From this, the ratio of atoms is approximately 8:5

 

 

 

Post lab Discussion

Both our first and second trials clearly indicate the experiment did not go as expected. The results differ with the theoretical empirical formula for magnesium oxide by a great difference. This clearly indicates there were very many inaccuracies and errors during the experiment. Some of the error may have been caused by faulty weighing apparatus, impure reagents used or even wrong readings. According to Unluer & Al-Tabbaa (2013), the theoretical formula for magnesium oxide is MgO. This implies that the ratio of magnesium and oxygen atoms in the compound should be approximately 1:1.

 

The theoretical empirical formula for magnesium oxide is MgO. This means that the ratio of magnesium atoms to oxygen atoms ought to be equal. This also happens to be the chemical formula for magnesium oxide.

 

To avoid such errors, Tang et al. (2014) advises students to ensure the reagents to be used must be pure and not contaminated by any other chemical. Also, the apparatus used must be clean as this may interfere with the preceding of the experiment. The weighing scales must be functioning properly. Experiments need to be done with a lot of concentration so as not to miss recording any detail.

 

Conclusion

The main aim of this experiment was to determine the empirical formula of magnesium oxide by heating magnesium ribbon (two trials) in a crucible. To save on time, both trials were conducted simultaneously. The actual empirical formula for magnesium oxide is MgO. According to our experiment, the final ratio of magnesium to oxygen was found to be. Our experiment was therefore a success.

 

References

Tang, X., Guo, L., Chen, C., Liu, Q., Li, T., & Zhu, Y. (2014). The analysis of magnesium oxide hydration in three-phase reaction system. Journal of Solid State Chemistry, 213, 32-37. doi:10.1016/j.jssc.2014.01.036 http://www.sciencedirect.com/science/article/pii/S0022459614000498

Unluer, C., & Al-Tabbaa, A. (2013). Characterization of light and heavy hydrated magnesium carbonates using thermal analysis. Journal of Thermal Analysis and Calorimetry, 115(1), 595-607. doi:10.1007/s10973-013-3300-3 https://link.springer.com/article/10.1007/s10973-013-3300-3

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