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AQA GCSE · Question 09.2 · Quantitative chemistry
Figure 11 shows the volumes of hydrogen collected in the 50 cm³ measuring cylinder after 40 seconds and after 100 seconds.
Determine the number of moles of hydrogen collected between 40 seconds and 100 seconds.
The volume of one mole of any gas at room temperature and pressure is 24 dm³.
Figure 11 shows the volumes of hydrogen collected in the 50 cm³ measuring cylinder after 40 seconds and after 100 seconds.
Determine the number of moles of hydrogen collected between 40 seconds and 100 seconds.
The volume of one mole of any gas at room temperature and pressure is 24 dm³.
How to approach this question
This is a multi-step calculation involving reading scales and using the molar volume of a gas.
1. **Read the volumes:** Carefully read the volume of gas collected in each measuring cylinder. Remember the scale is inverted, so you are reading the volume of the gas at the top, not the water level. The markings are every 2 cm³.
2. **Calculate the difference:** Subtract the volume at 40 s from the volume at 100 s to find the volume of gas produced *in that time interval*.
3. **Unit conversion:** The molar volume is given in dm³, but your measured volume is in cm³. You must convert your volume to dm³ by dividing by 1000.
4. **Calculate moles:** Use the formula: moles = volume in dm³ / 24.
Full Answer
**Step 1: Read the volumes from the measuring cylinders.**
- Volume at 40 s = 26 cm³
- Volume at 100 s = 48 cm³
**Step 2: Calculate the volume of gas collected between 40 s and 100 s.**
Volume collected = Volume at 100 s - Volume at 40 s
Volume collected = 48 cm³ - 26 cm³ = 22 cm³
**Step 3: Convert the volume from cm³ to dm³.**
Since 1 dm³ = 1000 cm³, we divide by 1000.
Volume in dm³ = 22 / 1000 = 0.022 dm³
**Step 4: Calculate the number of moles.**
Moles = Volume (in dm³) / Molar volume (24 dm³/mol)
Moles = 0.022 / 24
Moles = 0.00091666... mol
**Step 5: Round to an appropriate number of significant figures (e.g., 2 or 3).**
Moles = 0.000917 mol (to 3 s.f.) or 9.17 x 10⁻⁴ mol.
This calculation requires careful reading of the scales and correct application of the molar volume formula.
1. **Reading the Scales:** The measuring cylinders are upside down. The volume of gas is the space from the top down to the water level.
- At 40 seconds: The water level is at 26 cm³. (Each major marking is 10, each minor marking is 2).
- At 100 seconds: The water level is at 48 cm³.
2. **Calculating Volume Produced:** The volume of hydrogen collected between these two times is the difference between the final and initial volumes.
Volume = 48 cm³ - 26 cm³ = 22 cm³.
3. **Converting Units:** The molar volume of a gas is given as 24 dm³ per mole. Our volume is in cm³. To use the formula, we must convert our volume to dm³. Since 1 dm³ = 1000 cm³, we divide by 1000.
Volume = 22 cm³ / 1000 = 0.022 dm³.
4. **Calculating Moles:** The number of moles of a gas is calculated by:
Moles = Volume in dm³ / Molar Volume
Moles = 0.022 dm³ / 24 dm³/mol
Moles = 0.0009166... mol.
Rounding to 3 significant figures gives 0.000917 mol.
Common mistakes
✗ Misreading the scales on the measuring cylinders.
✗ Forgetting to find the difference between the two volumes.
✗ Forgetting to convert the volume from cm³ to dm³ before calculating moles.
✗ Dividing 24 by the volume instead of the other way around.
Practice the full AQA GCSE Chemistry Higher Tier Paper 2
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