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2.1States of Matter

Gases — EST 2 Chemistry Revision Notes & Topical Questions

5 min read 8 parts Lesson 1 of 3 1 quiz

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Lesson summary

Gases EST Chemistry notes: Apply Avogadro's principle.At STP one mole of any gas occupies about 22.4 L.Density of a gas = PM/RT.Ideal gases have no volume and no attractions; real gases have both.Helium and hydrogen behave most ideally.The ideal gas law combines all the simple ga…

  • How many moles of gas occupy 5.00 L at 2.00 atm and 300 K?
  • Gases EST Chemistry: what to do next
  • Molar volume
  • Ideal gas constant (R)
  • Identify / Describe (FRQ)
  • Explain / Justify (FRQ)

Section 1 of 8Gases

1
n is moles, so masses must be converted.
2
Use R = 0.0821 with atm, 8.314 with kPa.
3
Molar mass can be determined from gas measurements.
4
Real gases condense because of attractions.
5
Ideal behaviour is an approximation.

Section 2 of 8Process & sequence

1
Write PV = nRT.
2
Choose R to match the pressure units.
3
Convert temperature to kelvin and volume to litres.
4
Solve for the unknown.
5
Molar mass can be found from M = mRT/PV.

Using PV = nRTwrite these steps in this order; each step is a marking point.

  1. Write equation
  2. Choose R
  3. Convert units
  4. Solve
  5. Check sense

Section 3 of 8Compare

When gases behave ideally
ConditionIdeal?Reason
High temperature, low pressureClose to idealParticles far apart and fast
Low temperatureLess idealIMFs matter
High pressureLess idealParticle volume matters
Polar gasesLess idealStronger attractions

Section 4 of 8Worked examples

  1. How many moles of gas occupy 5.00 L at 2.00 atm and 300 K?

Section 5 of 8Keep going

Gases EST Chemistry: what to do next

These Gases EST Chemistry notes covered the key ideas, worked examples and exam technique for this topic. Check your Gases EST Chemistry understanding with the practice questions below, then reread any Gases EST Chemistry point you were unsure about before moving on.

1
Practice: Gases EST Chemistry practice questions
2
Previous lesson: 1.5 Nuclear chemistry
3
Next lesson: 2.2 Liquids and solids
4
Course: EST 2 Chemistry — all units
5
Official syllabus: OpenStax free science textbooks

Gases EST Chemistry revision notes

Section 6 of 8Key terms

Section 7 of 8Command terms

Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Section 8 of 8Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Avogadro's principle · Molar volume · STP · Ideal gas law · Ideal gas constant (R) · Real gas
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.

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