Lab Percentage Composition of Magnesium Oxide

Prelab: Determine the percentage composition of magnesium and oxygen in MgO from your periodic table.

Purpose: To determine the percentage composition of Mg and Oxygen by mass and determine the empirical formula.

Procedure and materials:

  1. Determine the mass of a clean dry crucible with its lid.
  1. Measure off about 15 cm of magnesium ribbon that has been cleaned with steel wool. Wind the ribbon around a pencil to form a loose spiral. Place the spiral of magnesium in the crucible and determine the mass of crucible, lid and magnesium ribbon.
  1. Support the crucible on a clay triangle on a ring stand. Place the lid on the crucible so that a small opening remains.
  1. Heat the crucible using a hot Bunsen burner flame for about eight minutes. Carefully slide the lid off with crucible tongs. If the contents start to burn with a bright white flame, replace the lid and continue to heat until the magnesium no longer flares up when the lid is removed. The contents should now be a crumbly grayish white powder.
  1. Remove the lid carefully onto a ceramic pad and allow the crucible to cool. Failure to allow the crucible to cool before weighing can both cause damage to the balance and an incorrect weight due to the buoyancy effect of hot rising air.
  1. When the crucible and contents are reasonably cool reweigh the crucible, lid and contents (magnesium oxide).

Data Table and Calculations

Mass of crucible and lid22.47+0.01g

Mass of crucible, lid and magnesium22.75+0.01g

Mass of crucible, lid and magnesium oxide22.93+0.01g

Mass of magnesium oxide produced0.46+0.01g

Mass of magnesium that reacted0.28+0.01g

Mass of oxygen that reacted with magnesium0.18+0.01g

Discussion and Analysis—show all work and use correct sig figs.

  1. Find the percentage composition of magnesium and oxygen from your data include uncertainty:

Mass of Mg = ______

Divided byX 100% = ______% Mg

Mass of MgO = ______

Mass of Oxygen = ______

Divided by X 100% = ______% O

Mass of MgO = ______

  1. Find the percentage error for each of the above from the calculated values from your periodic table. │(Actual-Observed)/(Actual)│x100 = % error

Percentage error for Mg

Percentage error for Oxygen

  1. Calculate the empirical formula of the compound using your masses from the data table.

Use the three steps. Find molesRatio

  1. Mass of Mg = ______
  1. Mass of oxygen= ______
  1. Empirical formula = ______
  1. When an electric current is passed through 60.0mL of water, a total of 53.33 grams of oxygen gas and 6.67g hydrogen gas are produced before all the water is decomposed. Calculate the percentage by mass of oxygen and hydrogen in water.