IUA-,TERIAL S AND E QUI PME NT Solids: Reagent Grade potassium chlorate (KC1O3)and potassium chloride (KCl). Solutions: dilute (6 M) nitric acid (HNO3) and 0.1M silvernitrate (AgNO3). Th.o No.0 crucibles with covers; Ceramfab pad. DISCUSSION The percentage composition of a compound is the percentage by mass of each element in the compound. If the formula of a compound is known, the percentage composition can be calculated from the molar mass and the total mass of each element in the compound. The molar mass of a compound is determined by adding up the atomic masses of all the atoms making up the formula. The total mass of an element in a compound is determined by multiplying the atomic mass of that element by the number of atoms of that element in the formula. The percentage of each element is then calculated by dividing its total mass in the compound by the molar mass of the compound and multiplying by 100. The percentage composition of many compounds may be directly determined or verified by experimental methods. In this experiment the percentage composition of potassium chlorate will be determined both experimentally and from the formula. When potassium chlorate is heated to high temperatures (above 400'C) it decomposes poto tassium chloride and elemental oxygen, according to the following equation: 2 KCIOB(s) -----+ 2 KCI(s) + 3 Ork) The relative amounts of oxygen and potassium chloride are measured by heating a weighed sample of potassium chlorate and determining the amount of residue (potassium chloride) remaining. The decrease in mass brought about by heating represents the amount of oxygen originally present in the sample. From the experiment we obtain the following three values: 1. Mass of original sample (KCIO3). 2. Mass lost when sample was heated (Oxygen). 3. Mass of residue (KCI). From these experimental values (and a table of atomic masses) we can calculate the following: 4. Percentage oxygen in sample (Experimental value) Mass lost by sample x 100 Original sample mass
5 . Percentage KCI in sample (Experimental value)
Mass of residue x 100 Original sample mass 6 . Percentage oxygen in KClo3 from formula (Theoretical value) = 3 at. masseJ jlexIggn 3 x I-6.00g >< ^ ' 1.00_ x 100 122.69
7 . Percentage KCI in KCIO, from formula (Theoretical value) Molar mass of KCI x l o o - 7 4 ' 5 5 sx l o o Molar mass of KCIO, 122.6 g 8. Percentage error in experimental oxygen determination _ (Experimental value) (Theoretical value) rz1AA Theoretical value
PRECAUTIONS: Since potassium chlorate is a strong oxidizing agent it may cause fires or explosions if mixed or heated with combustible (oxidizable) materials such as paper. Observe the following safety precautions when working with potassium chlorate: 1. Wear protective glasses.
2. Use clean crucibles that have been heated and cooledprior to adding potassium chlorate. 3. Use Reagent Grade potassium chlorate. 4. Dispose of any excess or spilled potassium chlorate as directed by your instructon (Potassium chlorate may start fires if mixed with paper or other solid wastes.) 5. Heat samples slowly and carefully to avoid spattering molten material-and poor experimental results. NOTES: 1. Make all weighings to the highest precision possible with the balance available to you. Use the same balance to make all weighings for a given sample. Record all data directly on the report sheet as they are obtained. 2. Duplicate samples of potassium chlorate are to be analyzed, if two crucibles are available. 3. For utmost precision, handle crucibles with tongs after the initial heating. to avoid
Place a clean, dry crucible (uncovered) on a clay triangle and heat for 2 or 3 minutes at the maximum flame temperature. The tip of the sharply defined inner-blue cone of the flame should almost touch and heat the crucible bottom to redness. Allow the crucible to...