9701 · 2.4
Reacting masses and volumes (of solutions and gases) flashcards
Revision flashcards for Cambridge 9701 Reacting masses and volumes (of solutions and gases) (syllabus 2.4). Flip, recall, then mark a real past-paper question.
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What is the formula linking moles (n), mass (m), and molar mass (Mr)?
$n = \frac{m}{M_r}$
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What is the molar gas volume at room temperature and pressure (RTP)?
24 dm³ mol⁻¹ (or 24,000 cm³ mol⁻¹). This is the volume occupied by one mole of any gas at RTP.
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What are the conditions for Room Temperature and Pressure (RTP)?
Approximately 20-25 °C (around 293-298 K) and 1 atmosphere (101 kPa) pressure. Cambridge exams will always provide the molar volume value to use.
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What is the formula linking moles (n), concentration (c), and volume (V)?
$n = c \times V$. Remember that volume must be in dm³.
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How do you convert a volume from cm³ to dm³?
Divide by 1000. For example, 250 cm³ = 0.250 dm³.
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What is a limiting reactant (or reagent)?
The reactant that is completely consumed first in a chemical reaction. It determines the maximum amount of product that can be formed.
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What is a reactant in excess?
A reactant that is not completely used up when the reaction finishes. Some of it will be left over.
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What is the first step in ANY reacting quantity calculation?
Write a balanced chemical equation for the reaction.
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In a titration, what is the 'titre'?
The final volume of solution added from the burette to reach the end-point. It's calculated by subtracting the initial burette reading from the final burette reading.
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Why is it important to use the mole ratio from the balanced equation?
It links the amount of one substance to another. Forgetting this step is a very common error. For $2A + B \rightarrow C$, the mole ratio of A to B is 2:1.
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What is theoretical yield?
The maximum amount of product that can be produced from the given amounts of reactants, calculated based on the limiting reactant.