Reacting gas volumes
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Flip to reveal answersState Avogadro's law of combining volumes.
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Question
State Avogadro's law of combining volumes.
Answer
At the **same temperature and pressure**, equal **volumes** of gases contain equal numbers of **moles** — so the volume ratio equals the coefficient ratio.
Question
Why can you use volume ratios directly for reacting gases?
Answer
Because at fixed T and P volume is **proportional to amount**, so the balanced **coefficients** give the **volume ratio** — no moles needed.
Question
What is the molar volume of a gas at STP?
Answer
**22.7 dm³ mol⁻¹** at STP (273 K, 100 kPa) — given in the data booklet.
Question
Formula linking amount and gas volume at STP?
Answer
$n = \dfrac{V}{V_{m}}$ with $V_{m} = 22.7$ dm³ mol⁻¹ (volume in dm³).
Question
How do you get a gas volume from an amount at STP?
Answer
Multiply the amount by the molar volume: $V = n\,V_{m} = n \times 22.7$ dm³.
Question
How many cm³ are in 1 dm³?
Answer
**1000 cm³** — divide a cm³ value by 1000 before using the molar volume 22.7 dm³ mol⁻¹.
Question
In N_{2} + 3H_{2} → 2NH_{3}, what volume of NH_{3} comes from 1 vol N_{2}?
Answer
**2 volumes** of NH_{3} (the volume ratio matches the 1 : 3 : 2 coefficients).
Question
How do you find the volume of an unreacted excess gas?
Answer
Subtract the volume that **reacted** (from the coefficient ratio) from the volume **supplied**.
Question
Does liquid water count in a 'total gas volume' answer?
Answer
**No** — only **gases** contribute; liquids and solids (like condensed water) add zero volume.
Question
Common reacting-gas-volume trap?
Answer
Forgetting to **subtract the gas that reacted** when asked for the volume remaining, or counting **liquid** products as gas.
Question
STP conditions for V_{m} = 22.7 dm³ mol⁻¹?
Answer
**273 K and 100 kPa** (standard temperature and pressure).
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Full study notes for Reacting gas volumes
Topic 5.1 hub
How much? The amount of chemical change
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