and Stoichiometry the Ideal Gas Law
By Yiliu (Peter) Wang Teaching Assistant: Josie Bodle October 19th, 2007
Results and Discussion:
An nitrate salt of the formula MNO2, where M+ is an alkali metal cation (Li+ , Na+ , K+ , Rb+) was reacted with sulfamic acid, HSO3NH2. Using the volume of the gas, the temperature of the gas, the pressure of the gas, we can figure out the moles of Nitrogen gas present using the ideal gas law. Then using the stoichiometric relationship between the moles of Nitrogen gas and the nitrate salt, we are able to figure out the identity of the alkali metal cation. Overall equation: MNO2 (aq) + HSO3NH2 (aq) -> MHSO4 (aq) + H2O (l) + N2 (g) 1. The mass of the solid MNO2 crystals was measured before it was mixed in with water. The mass of the test-tube was measured to be 3.536 grams, and the mass of the testtube and the nitrate salt sample was measured to be 4.305 grams. 4.305 grams 3.546 grams = .759 grams 2. Then mixed in MNO2 crystals with water (3 mLs) and then mixed it in with sulfamic acid, and the products are as shown in the overall reaction. Observation: nitrogen gas fizzed out constantly, fast at the beginning and then slower and slower as the nitrate salt was running out. This process occurred in the Erlenmeyer flask, and the Nitrogen gas exerted pressure on the body of water in the Florence flask, and thus the water pushed out to the beaker. This gives a relatively accurate measure of the amount of Nitrogen gas that was formed. The volume of water displaced (after the barometric pressure equalization, or step 4) was measured by its mass. 411.1 grams (mass of beaker + water) 191.900 grams (mass of beaker) = 219.2 grams (water) 219.2 grams of water X (1 mL of water / 1 g of water) X ( 1 mL of N2 / 1 mL of water) X (1 L / 1000 mL) = .2192 L N2 3. 2. The temperature in the Erlenmeyer flask approximated the temperature of the nitrogen gas. This temperature was measured right after the reaction stopped taking place.
Temperature: 28.0 o C + 273.15 = 301.15 K 4. The pressure of the N2 gas was then measured. First, the water levels are shifted so that they equal. both The water level in the beaker was affected by the barometric pressure, and the water level inside the Florence flask was affected by the pressure inside the Florence flask. When the two water levels even, the pressure of the Florence flask equals the barometric pressure, which is measured out to be 742 mmHg. The Florence flask contained vapor pressure and the pressure of the Nitrogen gas, and thus by using Dalton's Law of Partial pressures: 742 mmHg = PNitrogen Gas + Pwater's vapor pressure at 28 Celsius 742 mmHg 28.3 mmHg (vapor pressure of water at 28 Celsius) = 714 mmHg for Nitrogen gas. 714 mmHg X (1 atm / 760 mmHg) = .939 atm of nitrogen gas 5. With the values for pressure, volume, and temperature, the moles of N2 can be found. PV = nRT .939 atm X .2192 L = 0.0821 X 301.15 K X n n = .00832 moles of nitrogen gas 1:1 ratio of nitrogen gas compared to MNO2, thus .00832 moles of MNO2. 6. Mass of MNO2 = .759 grams Moles of MNO2 = .00832 moles Molar mass = .759 grams / .00832 moles = 91.2 grams/mole 91.2 grams/mole of MNO2 14.0 grams/ mole of N 32.0 grams/mole of O2 = 45.2 grams/ mole of M M approximates the value of Potassium the most, which is 39.10 grams/ mole.
Accuracy: percentage error = |(39.1 g/mole 45.2 g/mole) / 45.2 g/mole | X 100% = 13.5% Conclusion: the unknown M was approximated relatively accurately to be Potassium.
Sources of Error: first off, the law is `ideal', because it only works in ideal conditions, so deviations are predisposed to appear because conditions are never ideal. Secondly, the temperature was measured after the reaction took place, and the temperature of the gas (which is hard to measure) should differ slightly compared to the temperature of the water. Thirdly, the moles of N 2 in the container should still be different than the calculated mole amount through the displacement of water, because after the pressure levels off, the amount of Nitrogen gas still retains inside the Erlenmeyer flask and Florence flask. Finally, the reaction did not use all the nitrite salt because proceeding to completion is not possible in acting settings.