Safety Precautions:
- Wear safety glasses at all times to avoid potassium nitrate entering the eye. Should this occur however, immediately wash eyes with water for 10 minutes and seek medical help.
- Clean up any spillages immediately.
- Any skin contact with the substance, wash with water immediately.
- Wear a lab coat at all times to avoid contact with clothes etc.
- To reduce the risk of burns, always hold the Bunsen burner at the bottom, and leave all apparatus to cool before touching it.
- Take care when handling glassware. When sliding delivery tubes or test tubes into rubber tubing or when inserting bungs, always hold the glass as close as possible to the top of the object.
Diagram:
Determining the mass of KNO3 to use:
- Chose a volume of gas which will be easily measurable by the gas syringe but large enough to reduce its possible error -roughly ±1cm3, or 1.25% in this case.
-
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 ½ 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%. When added to the error created when reading the gas syringe, there is an altogether 8.66% error in the results which may need to be taken into consideration if results are off scale- however it should not be a problem in this experiment.
Method:
- Set up the gas syringe apparatus as shown in above diagram. Take care that the equipment is properly connected by pulling on the syringe- should feel elastic and on release returns to roughly the same position.
- Measure out exactly 0.27g of KNO3 into dry weighing bottle- taking care to set the scales to zero using Tare (T) before weighing the substance.
- Transfer the potassium nitrate into the test tube and insert the bung.
- Check that the gas syringe is fully depressed before starting heating
- Heat the test tube on a blue flame, moving the Bunsen back and forth over the part of the test tube holding the potassium nitrate.
- Heat this way until all the potassium nitrate has turned black and no more gas seems to be produced.
- 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) + ½ 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:
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If 0.001 moles of gas (32cm3) is given off, equation 1 is correct.
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If 0.003 moles of gas (80 cm3) is given off, equation 2 is correct.
Bibliography:
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- Hills Road, Bench Chemist guide, pages 11-14
- http://www.wpbschoolhouse.btinternet.co.uk/page04/4_73calcs10rgv.htm