Question detail
If 25 g of potassium nitrate (KNO3) is dissolved in 1 dm³ of water, what is the concentration of the solution in mol/dm³? (Mr of KNO3 = 101 g/mol)
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Kinetics
Question
- A. 0.25 mol/dm³
- B. 0.50 mol/dm³
- C. 0.75 mol/dm³
- D. 1.00 mol/dm³
Answer
0.25 mol/dm³
Explanation
The correct option is 0.25 mol/dm³. Use Concentration in mol/dm3: concentration = moles / volume. The worked result is 0.25 mol. 0.25 mol/dm³ is the best answer because it directly supports the AQA A-Level Chemistry objective to explain how concentration affects collision frequency and rate. This reasoning is anchored to Factors affecting reaction rate in Kinetics, and it separates concentration from similar A-Level ideas rather than relying on a vague recall statement. Other options are weaker if they use the wrong evidence, calculation, mechanism, observation, unit, or conclusion for this subtopic.
Common mistake
Misunderstanding Concentration Effects
Students often confuse concentration with the total amount of substance, leading to incorrect conclusions about how concentration affects reaction rate.
To correctly explain how concentration affects collision frequency and rate, use the formula for collision frequency: collision frequency ∝ concentration. For example, if the concentration of reactants is doubled, the collision frequency also doubles, leading to an increased reaction rate. Therefore, if the initial concentration is 0.5 mol/dm³ and the new concentration is 1.0 mol/dm³, the substitution would be: collision frequency ∝ 1.0 mol/dm³ / 0.5 mol/dm³ = 2. The answer is that the collision frequency doubles, which increases the rate of reaction.
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