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Nitrous acid (HNO₂) is a weak acid that is readily oxidised to the strong acid, nitric acid (HNO₃) - Leaving Cert Chemistry - Question 7 - 2014
Question 7
Nitrous acid (HNO₂) is a weak acid that is readily oxidised to the strong acid, nitric acid (HNO₃). (a) Distinguish between a strong acid and a weak acid according ... show full transcript
Worked Solution & Example Answer:Nitrous acid (HNO₂) is a weak acid that is readily oxidised to the strong acid, nitric acid (HNO₃) - Leaving Cert Chemistry - Question 7 - 2014
Step 1
Distinguish between a strong acid and a weak acid according to the Brønsted-Lowry theory.
Answer
In the Brønsted-Lowry theory, a strong acid is defined as a substance that is a good proton donor, meaning it readily donates protons (H⁺ ions) to other substances. On the other hand, a weak acid is characterized as a poor proton donor, which means it does not completely dissociate in solution and has a low tendency to donate protons.
Step 2
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Step 5
Explain clearly how this H₃O⁺ ion concentration confirms that nitrous acid is a weak acid.
Answer
The concentration of H₃O⁺ ions at 0.01 M indicates that only a small fraction of the nitrous acid has dissociated in solution. This low concentration of ions, relative to the original concentration of the acid, confirms that nitrous acid is a weak acid because strong acids completely ionize in solution, resulting in higher concentrations of H₃O⁺.
Step 6
What concentration of nitric acid would have the same H₃O⁺ ion concentration?
Answer
A strong acid like nitric acid (HNO₃) would completely dissociate in solution, thus a concentration of nitric acid that results in the same H₃O⁺ concentration of 0.01 M would also be 0.01 M, as it completely ionizes.
Step 7
Calculate the OH⁻ ion concentration in both of these acidic solutions.
Answer
Using the relationship between pH and pOH: For nitrous acid at pH 2.0:
[ ext{OH}^-] = 10^{-pOH} = 10^{-12} ext{ M}$$ The concentration of hydroxide ions in the nitric acid solution would be the same at 0.01 M H₃O⁺, resulting in: $$pOH = 14 - 2.0 = 12 \\ [ ext{OH}^-] = 10^{-12} ext{ M}$$Step 8
Describe in detail how you could test for the presence of the nitrate anion in aqueous solution.
Answer
To test for the presence of the nitrate ion (NO₃⁻) in aqueous solution, one could perform the following test:
- Add freshly prepared iron(II) sulfate (FeSO₄) solution to the sample.
- Gently heat the mixture while observing for a brown ring that forms at the junction of the two liquids, indicating the presence of the nitrate ion. This test is based on the reaction of nitrate ions with iron(II) ions to form a complex that gives a characteristic color change.
Step 9
Explain how high nitrate levels can result in a reduction in the dissolved oxygen content of lakes and rivers.
Answer
High nitrate levels in aquatic ecosystems can lead to eutrophication. This process causes rapid growth of water plants and algae, which upon decomposition consume oxygen in the water. Consequently, lower levels of dissolved oxygen can harm aquatic life, disrupting ecosystems and potentially leading to dead zones where aquatic life cannot survive.