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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 app... 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 ext{KOH} + ext{H}_2$$ This indicates that two moles of potassium react with two moles of water to produce two moles of potassium hydroxide and one mole of hydrogen gas.Step 2
Give two differences you would see between the reactions of potassium and lithium with water.
Answer
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Reaction Vigour: Potassium reacts more vigorously than lithium, producing more gas and bubbling rapidly.
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Flame Color: When potassium reacts, it produces a lilac flame, whereas lithium does not produce a distinctive flame.
Step 3
Describe how a student could do titrations to find the mean volume of potassium hydroxide solution which would neutralise 25.00 cm³ of nitric acid.
Answer
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Prepare the solution: Fill a burette with the potassium hydroxide solution.
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Add acid to a flask: Measure 25.00 cm³ of nitric acid into a flask.
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Add indicator: Add a few drops of a suitable indicator (such as phenolphthalein) to the nitric acid.
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Perform the titration: Slowly add the potassium hydroxide from the burette to the acid while swirling the flask, until the indicator changes color.
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Record the volume: Take a reading of the volume of potassium hydroxide used from the burette.
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Repeat for accuracy: Repeat the titration process multiple times to find concordant results and calculate the mean volume.
Step 4
Calculate the concentration of the nitric acid.
Answer
Using the equation:
ightarrow ext{KNO}_3 + ext{H}_2 ext{O} $$ We know that 26.25 cm³ of potassium hydroxide solution with a concentration of 0.20 mol/dm³ neutralises 25.00 cm³ of nitric acid. First, convert the volume of KOH to dm³: $$26.25 ext{ cm}^3 = 0.02625 ext{ dm}^3 $$ Now, calculate the number of moles of KOH used: $$ ext{Moles of KOH} = ext{Concentration} imes ext{Volume} = 0.20 ext{ mol/dm}^3 imes 0.02625 ext{ dm}^3 = 0.00525 ext{ moles}$$ As the reaction is a 1:1 ratio, the moles of nitric acid will also be 0.00525 moles. To find the concentration of nitric acid: $$ ext{Concentration of HNO}_3 = rac{ ext{Moles}}{ ext{Volume}} = rac{0.00525 ext{ moles}}{0.025 ext{ dm}^3} = 0.21 ext{ mol/dm}^3$$