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A sample of mixed alkali (naoh / NaHCO3/ Na2CO3) weighing 3.216 g was dissolved in 250- ml CO2-free water. a 20-ml aliquot of the solution required 54.40 ml of 0.0581m HCl titrant to reach the phenolphthalein endpoint. Bromocresol green was then added and an additional 27.40 ml of titrant. What is/are the component? a) NaHCO3 B) Na2CO C) NaOH and Na2CO D) NaHCO3 and Na2CO3
Accepted Answer
The correct answer is D) NaHCO3 and Na2CO3. Here's why:
Phenolphthalein Endpoint: This endpoint indicates the neutralization of strong base (NaOH) and the first proton of carbonate (Na2CO3) to form bicarbonate (NaHCO3).
Bromocresol Green Endpoint: This endpoint indicates the neutralization of the second proton of carbonate (NaHCO3) to form carbonic acid (H2CO3).
Let's analyze the titration steps:
1. First Titration: The 54.40 mL of HCl reacts with NaOH and the first proton of Na2CO3. This reaction is represented by:
NaOH + HCl → NaCl + H2O
Na2CO3 + HCl → NaHCO3 + NaCl
2. Second Titration: The additional 27.40 mL of HCl reacts with the remaining NaHCO3. This reaction is represented by:
NaHCO3 + HCl → NaCl + H2O + CO2
Since we have two distinct titration points, it confirms the presence of both NaHCO3 and Na2CO3 in the sample.
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