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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. (a)(i) Potassium reacts with water to produce potassium hydroxide solution and a gas. Figure 8 shows the appara... 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 shows 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 rapidly with water compared to lithium, producing a larger amount of gas and heat.
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Flame Color: Potassium produces a lilac flame during the reaction, while lithium produces a crimson 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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Preparation: Fill a burette with the potassium hydroxide solution and place a conical flask beneath it containing the nitric acid solution mixed with a few drops of an appropriate indicator.
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Titration Process: Open the burette tap to allow potassium hydroxide to flow into the acid solution, swirling the flask continuously.
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Observation: Stop adding the alkali when the indicator changes color, signifying that the solution is neutralized.
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Record Volumes: Take note of the initial and final volume reading on the burette to calculate the volume of potassium hydroxide used.
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Repeat: Repeat the titration process several times and calculate the mean of the volumes recorded.
Step 4
Calculate the concentration of the nitric acid.
Answer
Given that 26.25 cm³ of potassium hydroxide solution with a concentration of 0.20 moles per dm³ neutralises 25.00 cm³ of nitric acid:
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Calculate moles of KOH used:
ext{Moles of KOH} = ext{Concentration} imes ext{Volume} = 0.20 ext{ mol/dm}^3 imes rac{26.25}{1000} = 0.00525 ext{ moles}
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From the reaction ratio:
ext{KOH} + ext{HNO}_3 ightarrow ext{KNO}_3 + ext{H}_2 ext{O}
This shows that moles of KOH = moles of HNO₃, so 0.00525 moles of HNO₃ were neutralized.
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Calculate concentration of HNO₃:
ext{Concentration} = rac{ ext{Moles}}{ ext{Volume}} = rac{0.00525}{ rac{25}{1000}} = 0.21 ext{ mol/dm}^3.