Decomposing baking soda

Performance objectives To determine the percent yield of sodium carbonate from a decomposition reaction. To determine the percentage of sodium hydrogen carbonate in an unknown mixture. To gain proficiency in decomposing a compound and collecting a gas over water.

Discussion Heating sodium hydrogen carbonate, NaHCO3, produces sodium carbonate, steam and carbon dioxide gas respectively. Balance chemical equation: 2 NaHCO3(s) ----(heat) Na2CO3(s) + H2O(g) + CO2(g) The balance equation shows the release of H2O(g) and CO2(g) leaving Na2CO3(s)

discussion The product Na2CO3 is the mass collected and weighed after the reaction. This is referred to as the actual yield. The calculated mass of Na2CO3 from the balanced equation is the theoretical yield. The ratio of actual yield to theoretical yield from the balance equation times 100%. This is called the percent yield. It can be greater than or less than 100%.

Discussion Percent yield of Na2CO3 from Baking Soda – example problem 1 A 1.654 g sample of baking soda, NaHCO3, decomposes to produce 1.028 g of solid sodium carbonate. Calculate the theoretical yield and percent yield of Na2CO3. 1.654 g NaHCO3 x 1 mol NaHCO3/84.01 g NaHCO3 x 1 mol Na2CO3/2 mol NaHCO3 x 105.99 g Na2CO3/1 mol Na2CO3 = 1.043 g Na2CO3 actual/theoretical x 100% = % yield 1.028 g/1.043 g x 100% = 98.56 %

discussion Percent yield in an Unknown Mixture – example problem 2 A 1.675 g unknown mixture containing baking soda is decomposed with heat. If the mass loss is 0.318 g, what is the percentage of baking soda, NaHCO3, in the unknown mixture? Chemical equation: 2 NaHCO3(s) ---- (heat) Na2CO3(s) + H2CO3(g) 0.318 g H2CO3 x 1 mol H2CO3/62.03 g H2CO3 x 2 mol NaHCO3/1 mol H2CO3 x 84.01 g NaHCO3/1 mol NaHCO3 = 0.861 g NaHCO3 Sample mass = 1.675 g Mass NaHCO3/mass sample x 100% = %NaHCO3, 0.861 g/1.675 g x 100% = 51.4%

procedure Percent Yield of Na2CO3 from Baking Soda Weigh a 16 x 150 mm dry test tube on the balance, and record the mass. Add 1-2 g of baking soda, NaHCO3, and reweigh accurately. 2. Set up the apparatus as shown in Figure 1. Fill the Florence flask to the neck with tap water, and insert the gas collection apparatus. Insert the small rubber stopper into the test tube as shown.

procedure 3. Begin heating the test tube gently. Observe the water being displaced into the beaker as carbon dioxide gas is produced. As the water level in the beaker increases, continue to heat the test tube with a gentle flame. After the water level remains constant for a couple of minutes, discontinue heating and allow the test tube to cool for 10 minutes. 4. Weigh the test tube containing the sodium carbonate residue. The mass of Na2CO3 is found by subtracting the mass of the test tube from the test tube and residue.

procedure 5. Calculate the theoretical yield of sodium carbonate, Na2CO3, from the mass of pure baking soda that was heated. Find percent yield of sodium carbonate.

procedure B. Percentage of NaHCO3 in an Unknown Mixture Obtain an unknown sample containing baking soda. Record the unknown number in the Data Table. 2. Repeat steps 1-5 as in Procedure A, substituting the unknown mixture for the pure baking soda. 3. Calculate the mass of baking soda, NaHCO3, in the unknown sample from the mass loss. Find the percentage of baking soda in the unknown mixture.

Equipment Florence flask 1000 ml beaker Ring stand 16x150 mm test tube Bunsen burner Glass tubing