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The diagram shows three reagent bottles containing acids - HSC - SSCE Chemistry - Question 24 - 2004 - Paper 1
Question 24
The diagram shows three reagent bottles containing acids. pH = 3.38 0.01 mol L⁻¹ Acetic acid pH = 2.56 0.01 mol L⁻¹ Citric acid pH = 2.00 0.01 mol L⁻¹ Hydroc... show full transcript
Worked Solution & Example Answer:The diagram shows three reagent bottles containing acids - HSC - SSCE Chemistry - Question 24 - 2004 - Paper 1
Step 1
Calculate the pH after 10.0 mL of 0.01 mol L⁻¹ hydrochloric acid solution is diluted by the addition of 90.0 mL of distilled water.
Answer
To calculate the pH after dilution, we first need to determine the moles of hydrochloric acid. Given:
- Volume of HCl = 10.0 mL = 0.010 L
- Concentration of HCl = 0.01 mol L⁻¹
Moles of HCl = Volume × Concentration = 0.010 L × 0.01 mol L⁻¹ = 0.0001 moles
After dilution with 90.0 mL of distilled water, the total volume becomes 100.0 mL = 0.100 L.
The new concentration of hydrochloric acid after dilution is:
Concentration = Moles / Volume = 0.0001 moles / 0.100 L = 0.001 mol L⁻¹
Since HCl is a strong acid, it fully dissociates in solution. Therefore,
pH = -log[H⁺] = -log[0.001] = 3.00.
Step 2
Explain the use of acetic acid and citric acid as food additives.
Answer
Acetic acid and citric acid are commonly used food additives for several reasons:
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Preservation: Both acids lower the pH of food products, which helps inhibit the growth of bacteria and other pathogens, thus extending shelf life.
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Flavoring: They provide a sour taste, enhancing the flavor profile of various foods and beverages.
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pH Regulation: They can act as pH regulators, stabilizing the acidity of food products which is essential for certain chemical reactions and overall quality.
Step 3
Explain the difference in pH between the three acids in the diagram.
Answer
The pH difference among the three acids – acetic acid, citric acid, and hydrochloric acid – can be attributed to their strength and concentration:
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Hydrochloric Acid (HCl): As a strong acid, HCl fully dissociates in solution, resulting in a lower pH of 2.00, indicating a higher concentration of hydrogen ions.
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Citric Acid (C₆H₈O₇): Citric acid is a weak acid and does not fully dissociate. Its pH of 2.56 reflects a lesser concentration of hydrogen ions compared to HCl.
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Acetic Acid (CH₃COOH): Similarly, acetic acid is a weak acid with a pH of 3.38, indicating an even lower concentration of hydrogen ions. The differences in pH highlight the varying strengths and dissociation characteristics of these acids.