# Which is the correct equation?

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Introduction

Chemistry Skill P: Which is the correct equation? Potassium nitrate (KNO3) is a crystalline, white compound which is prepared commercially by reacting potassium chloride and sodium nitrate. Its uses in industry range from acting as a food preservative, to use in the manufacturing of explosives and fertilisers. This plan is to test which of the following two equations are accurate in measuring the gas products formed from heating KNO3: * 2KNO3(s) ----------> 2K02(s) + 02(g) * 2KNO3(s) ----------> K20(s) + 2NO2(g) + 1/2 02(g) It is noticeable that a different number of moles of gas are produced on heating potassium nitrate in the two equations. To determine which is correct, a calculated amount of potassium nitrate will be heated, and the volume of gas given off will be measured. The number of moles of gas calculated from this volume will provide the answer. Equipment: * Gas syringe and holder * Retort stand and boss * Additional boss and clamp * Spatula * Glass test tube with attached delivery tube and bung * Heat proof ...read more.

Middle

- 80cm3 --> 0.08dm3 2. Use equation: no. moles of gas = volume (dm3) / volume of one mole - 0.08dm3 / 24 = 0.003333moles of gas Equation 2 gives off more gas with 2 1/2 moles being produced by 2 moles of potassium nitrate in the ratio 4:5. The mass of potassium nitrate used will therefore be based on equation 2. - ((0.08/24) / 5) x 4 = 0.0026664 moles of KNO3 3. Use equation: no. moles = Mass (grams) / Relative atomic mass - 0.0026664 x 101.1 = 0.269573 grams As a top pan balance measures to two decimal places, the second decimal place will not be 100% reliable, resulting in an error of �0.01g. - Answer to 2 decimal places = 0.27grams. * To work out the percentage uncertainty: - Use the equation: - (0.001 / mass of KNO3) x 100% - (0.01 / 0.27) x 100 = 3.70% This results in an error of 7.40%. ...read more.

Conclusion

* Let the apparatus cool down before measuring the volume of gas. When the syringe plunger stops moving and the gas has finished contracting, then measure volume. * Repeat the experiment three times in order to find an average result and remove any anomalous data. Using the Results To find the number of moles of gas given off use the following equation: - Volume (dm3) / 24 (dm3) = No. of moles - 0.003 moles of potassium nitrate was used Equation 1: 2KNO3(s) ----------> 2K02(s) + 02(g) Equation 2: 2KNO3(s) ----------> K20(s) + 2NO2 (g) + 1/2 02(g) Ratio of moles potassium nitrate to gas produced: Equation 1 --> 2:1 So it takes 2 moles of KNO3 to produce one mole of gas - ((0.08/24) /5 x 2) x 24 = 0.032 dm3 Equation 2 --> 4:5 Likewise it takes 4 moles of KNO3 to produce 5 moles of gas - (0.08/24) x 24 = 0.080 dm3 Therefore: * If 0.001 moles of gas (32cm3) is given off, equation 1 is correct. * If 0.003 moles of gas (80 cm3) is given off, equation 2 is correct. ...read more.

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