Percent Composition and Moles

• The experiment aims to determine the percent composition of a mixture
• The experiment aims to differentiate between a compound and a mixture
• The experiment aims to perform basic laboratory skills

Materials Needed

• Ring stand
• Bunsen burner
• Tongs
• Clay tile
• Safety goggles
• Magnesium ribbon (Mg)
• Balance (to 0.0001g)
• Ring support/ clay triangle
• Crucible/lid

Theory

• A chemical formula determined by the mass of each element present in compound samples can be done by considering their relative atomic masses
• Mole: the unit of chemical quantity, which is the formula mass of a substance expressed in grams, not mass.
• A mole of any substance can be weighed out
• A mole of any substance contains the same number (Avogadro's number, 6.022 x 10^23) of formula units.
  • A mole of Uranium weighs 238.029 g and contains Avogadro's number of uranium atoms.
  • One mole of H2O weighs 18.015 g and contains Avogadro's number of H2O molecules.
  • One mole of NaCl weighs 58.443 g and contains Avogadro's number of NaCl formula units (NaCl ion pairs).
• Molar ratio: Dividing the mass of each element by the mass of one mole of the element gives the moles of each element present. When simplified to small whole numbers, this gives the empirical formula of the compound.
• Molecular formula: Gives the number of each kind of atom in a molecule of a compound and can be identical to or a multiple of the empirical formula, and to obtain the molecular formula from the empirical formula, the molecular weight of a compound must be known
  • Hydrogen peroxide's molecular weight is 34, and its empirical formula is HO. This means the hydrogen peroxide molecule contains equal numbers of hydrogen and oxygen atoms).
  • Hydrogen peroxide's molecular formula is H2O2 which corresponds to 34 Daltons.
• To find the empirical formula of a sulfide of copper, 1.956 g of copper wire is heated with a large excess of elemental sulfur to ensures that all the copper reacts.
  • Unreacted sulfur burns off to leave behind copper sulfide weighing 2.477 g.
  • Copper wire mass: 1.956 g
  • Copper sulfide mass (formed): 2.447 g
  • Combined sulfur mass: 0.491 g
• The molar of each element is derived from the equality as follows: one mol of any element = the atomic mass of the element expressed in grams
  • 1. 956g Cu x (1 mol Cu / 63.546 g Cu) = 3.078 x 10^-2 mol Cu
  • 2. 491gS x (1 mol S / 32.066 gS) = 1.53x10^-2 molS
• The molar amounts of each element depend on the amount of copper starting material
• To find the smallest whole number molar ratio by dividing both molar amounts by the smaller of the two values.
  • (3.078 x 10^-2 mol Cu) / (1.53 x 10^-2) = 2.01 mol Cu
  • (1.53 x 10^-2 molS) / (1.53 x 10^-2) = 1.00 mols
• The molar ratio of the two elements is close to 2:1, so the empirical formula of the compound must be Cu2S.
• The empirical formula Cu2S is an ion compound, and there are no molecules present, so the question of a molecular formula does not arise.
• Since percent composition is a common way of reporting analytical results, calculating percent compositions from analytical data is important
• Mass fraction: The mass of an element in the sample divided by the total sample mass.
  • mass fraction x 100% = percent of that element in the compound.
  • (1.956 g Cu / 2.447 g of compound) x 100% = 79.93% Cu by mass for the 2.447 g sample of Cu2S
  • (0.491gS / 2.447 g of compound) x 100% = 20.1% S by mass

Experimental Procedure

1. Fire the empty crucible and lid for about 3 minutes to remove water, oils, or other contaminants and to make sure there are no cracks. The bottom of the crucible should glow red-hot for about 20 seconds. Remove the flame and cool the crucible with lid.
2. The mass of the cooled crucible recorded should be accurate to ±0.0001 g, and it should not be handled without protection
3. Obtain about 0.3 g (35 mm) magnesium ribbon (do not handle the ribbon with your hands). Fold the ribbon to fit into the bottom of the crucible.
4. Record the mass of the magnesium ribbon and crucible to ±0.0001 g.
5. Place the crucible securely on the clay triangle. Set the lid slightly off-center on the crucible to allow air to enter but to prevent the magnesium oxide from escaping.
6. Place the Bunsen burner under the crucible, light it, and brush the bottom of the crucible with the flame for about 1 minute; then, place the burner under the crucible and heat strongly.
7. Heat until all the magnesium turns into gray-white powder (probably around 10 minutes).
8. Stop heating and allow the crucible, lid and contents to cool.
9. Add about 1 ml (~10 drops) of deionized water directly to the solid powder. Carefully waft some of the gas that is generated toward your nose but be very careful. Record any odor. Also, use tongs to carefully hold a piece of moist pH paper over the crucible. Record your observations.
10. Heat the crucible and contents, with the lid slightly ajar, gently for about 2 minutes and then strongly for about another 5 minutes.
11. Allow the crucible to cool and then record the mass of the crucible and contents to ±0.0001 g.

Notes on Magnesium Oxide Preparation

• The magnesium oxide preparation can be safely done on an open laboratory bench.
• Magnesium metal corrodes porcelain, so it is preferable to use old crucibles.
• Magnesium should be covered during burning to avoid losing magnesium oxide.
• Covers limit the reaction rate and capture most of the airborne magnesium oxide.

Safety

• Chemical splash goggles and a waterproof apron must be worn at all times during this and all chemistry experiments in the laboratory, from the very beginning to the very end of your time in the laboratory.

Disposal

• Magnesium oxide product and sand should be disposed of in the heavy metal hazardous waste container.

Cleanup

• Use a test tube brush and soapy water to wash the crucibles and rinse them with tap water.
• Keep one crucible in your laboratory locker and return all others to the laboratory supply area.
• All work surfaces should be wiped down with a damp sponge at the end of the lab period.

Crucible Use

• Crucibles are used to heat substances to high temperatures (like those encountered with burning metals) without risk of breakage.
• Crucibles are ceramic and can break; so be careful using them
• If a crucible breaks, inform a TA for clean-up and disposal.
• The crucible is now chemical waste and must be placed in the solid waste container.
• Avoid touching a hot crucible with bare hands to avoid contamination and burns.
• Place a hot crucible on the clay triangle instead of a lab bench to prevent breakage.

Prior Steps

• Practice using the tongs to pick up the lid from the crucible and the crucible from the clay triangle.
• Practice placing the lid partially over the crucible so that there is a gap of about 0.5 cm, where the lid should rest on the crucible edge and two legs of the triangle.
• Practice placing the crucible with lid on the clay tile, supporting it with tongs and the tile.

Magnesium Oxide Determination Data

• Trail Number
• Mass Of Mg
• Mass Of Crucible
• Mass Of Crucible + Mg
• Theoretical Yield Of MgO
• Mass Of MgO Product Formed To ±0.0001 G
• Mass of O
• Mole Ratio Of Mg-To-O
• Empirical Formula Of The Oxide
• Percent By Mass Of Mg And O In The Oxide
• Percent Yield

Guided Questions

1. What difference is there between an empirical and molecular formula?
2. What is the proper empirical formula for the compound ClO3.5?
3. A compound has the empirical formula CH2O and a molecular weight of 150.0 Daltons. What is the molecular formula?
4. Calculate the mass of 1.00 mole of F2.
5. What are two hazards associated with the use of concentrated nitric acid?
6. What is the empirical formula of a compound that consists of 69.9 % Fe and 30.1 % O and show calculated steps?