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This question is about potassium and its compounds - AQA - GCSE Chemistry - Question 4 - 2017 - Paper 3
Question 4
This question is about potassium and its compounds. 4 (a) (i) Potassium reacts with water to produce potassium hydroxide solution and a gas. Figure 8 shows the ... show full transcript
Worked Solution & Example Answer:This question is about potassium and its compounds - AQA - GCSE Chemistry - Question 4 - 2017 - Paper 3
Step 1
Complete and balance the equation for the reaction.
Answer
The balanced equation for the reaction of potassium with water is:
ightarrow 2 KOH + H_2$$ This indicates that two moles of potassium react with two moles of water to produce two moles of potassium hydroxide and one mole of hydrogen gas.Step 2
Give two differences you would see between the reactions of potassium and lithium with water.
Answer
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Reactivity: Potassium reacts more vigorously and quickly with water compared to lithium, producing a larger amount of gas.
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Flame Color: During the reaction, potassium produces a lilac flame due to the presence of potassium ions, while lithium does not.
Step 3
Describe how a student could do titrations to find the mean volume of potassium hydroxide solution which would neutralise 25.00 cm³ of nitric acid.
Answer
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Setup: Fill a burette with the potassium hydroxide solution. Use a pipette to measure exactly 25.00 cm³ of nitric acid into a conical flask.
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Add Indicator: Add a few drops of a suitable pH indicator, such as phenolphthalein, to the nitric acid solution in the conical flask.
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Titration: Slowly add the potassium hydroxide solution from the burette to the nitric acid while swirling the flask until the indicator changes color.
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Record Volume: Take note of the final volume of potassium hydroxide used at the end of the titration.
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Repeat: Repeat the titration to determine a consistent mean volume, recording all relevant measurements.
Step 4
Calculate the concentration of the nitric acid.
Answer
To calculate the concentration of nitric acid:
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Determine Moles of KOH:
Volume of KOH = 26.25 cm³ = 0.02625 dm³
Concentration of KOH = 0.20 moles/dm³
Moles of KOH = concentration × volume = 0.20 × 0.02625 = 0.00525 moles -
Reaction Stoichiometry: From the balanced equation, 1 mole of KOH reacts with 1 mole of HNO₃, so moles of HNO₃ = 0.00525 moles.
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Calculate Concentration of HNO₃:
Volume of HNO₃ = 25.00 cm³ = 0.02500 dm³
Concentration = moles/volume = 0.00525 moles / 0.02500 dm³ = 0.21 moles/dm³.