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15240672380's Question
Chemistry
Posted 10 months ago

Calculate volume of 0,1M0,1 \mathrm{M} ethanoic acid CH3COOH\mathrm{CH} 3 \mathrm{COOH} and 0,1M0,1 \mathrm{M} sodium ethanoate CH3\mathrm{CH} 3 COONa needed to prepare 100ml100 \mathrm{ml} of buffer with pH4,50!Pleasekindlyexplainand\mathrm{pH} \mathrm{4,50!} \mathrm{Please} \mathrm{kindly} \mathrm{explain} \mathrm{and} show(!!) how do the calculations! Thanks so much!
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Answer from Sia
Posted 10 months ago
Solution
1
Use the Henderson-Hasselbalch equation: The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation, which is given by pH=pKa+log([A][HA])\text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]}\right), where pKa\text{pKa} is the acid dissociation constant, [A][\text{A}^-] is the concentration of the conjugate base, and [HA][\text{HA}] is the concentration of the acid
2
Calculate the ratio of concentrations: For ethanoic acid, pKa=4.75\text{pKa} = 4.75. Given that the desired pH is 4.50, we can rearrange the Henderson-Hasselbalch equation to solve for the ratio [CH3COO][CH3COOH]\frac{[\text{CH}_3\text{COO}^-]}{[\text{CH}_3\text{COOH}]}: pH=pKa+log([CH3COO][CH3COOH])\text{pH} = \text{pKa} + \log \left( \frac{[\text{CH}_3\text{COO}^-]}{[\text{CH}_3\text{COOH}]}\right), which gives log([CH3COO][CH3COOH])=pHpKa=4.504.75=0.25\log \left( \frac{[\text{CH}_3\text{COO}^-]}{[\text{CH}_3\text{COOH}]}\right) = \text{pH} - \text{pKa} = 4.50 - 4.75 = -0.25. Therefore, [CH3COO][CH3COOH]=100.25\frac{[\text{CH}_3\text{COO}^-]}{[\text{CH}_3\text{COOH}]} = 10^{-0.25}
3
Determine the volumes of acid and conjugate base: Since the total volume of the buffer is 100 mL and the concentrations of both the acid and the conjugate base are 0.1 M, we can use the ratio from step 2 to find the volumes. Let VacidV_{\text{acid}} be the volume of ethanoic acid and VbaseV_{\text{base}} be the volume of sodium ethanoate. We have Vacid+Vbase=100V_{\text{acid}} + V_{\text{base}} = 100 mL and VbaseVacid=100.25\frac{V_{\text{base}}}{V_{\text{acid}}} = 10^{-0.25}. Solving these equations gives us Vacid=1001+100.25V_{\text{acid}} = \frac{100}{1 + 10^{-0.25}} mL and Vbase=100VacidV_{\text{base}} = 100 - V_{\text{acid}} mL
4
Calculate the exact volumes: Plugging the values into the equations from step 3, we get Vacid56V_{\text{acid}} \approx 56 mL and Vbase44V_{\text{base}} \approx 44 mL
Answer
To prepare 100 mL of a buffer with pH 4.50 using 0.1 M ethanoic acid and 0.1 M sodium ethanoate, you need approximately 56 mL of ethanoic acid and 44 mL of sodium ethanoate.
Key Concept
Henderson-Hasselbalch equation
Explanation
The Henderson-Hasselbalch equation relates the pH of a buffer solution to the pKa of the acid and the ratio of the concentrations of the conjugate base and the acid. By rearranging this equation, we can calculate the necessary volumes of acid and conjugate base to create a buffer with a specific pH.

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