A student is asked to determine the acid dissociation constant, Kₐ, for butanoic acid, CH3CH2CH2COOH. Kₐ = [CH3CH2CH2COO⁻][H⁺] / [CH3CH2CH2COOH] The student is told to measure the pH of eight buffer solutions containing CH3CH2CH2COOH and the salt sodium butanoate, CH3CH2CH2COO⁻Na⁺. The salt provides butanoate ions, CH3CH2CH2COO⁻, as the base. Each buffer solution contains a different ratio of [CH3CH2CH2COOH] to [CH3CH2CH2COO⁻]. Each buffer solution is prepared by mixing different volumes of distilled water, 2.00 mol dm⁻³ CH3CH2CH2COOH and 2.00 mol dm⁻³ CH3CH2CH2COO⁻Na⁺.

1. (a)Pure butanoic acid must be kept away from naked flames. Explain why.[1]
2. (b)(i)What is the maximum volume of a 2.00 mol dm⁻³ solution that can be prepared using 55.0 g of CH3CH2CH2COO⁻Na⁺(s)? [Aʳ: Na, 23.0; O, 16.0; C, 12.0; H, 1.0] volume = ....................................... cm³[1]
3. (b)(ii)A student is given 55.0 g of CH3CH2CH2COO⁻Na⁺(s) in a beaker. Describe the next steps the student should take to make a 2.00 mol dm⁻³ solution of the volume calculated in (b)(i). Give the name and capacity of any key apparatus which should be used. Write your answer as a series of numbered steps.[3]
4. (b)(iii)Complete the table to show the volumes of CH3CH2CH2COO⁻Na⁺(aq) and distilled water that would be needed to provide the stated number of moles of CH3CH2CH2COOH and CH3CH2CH2COO⁻ for a 50.0 cm³ buffer solution. | moles of CH3CH2CH2COOH | volume of 2.00 mol dm⁻³ CH3CH2CH2COOH /cm³ | moles of CH3CH2CH2COO⁻ | volume of 2.00 mol dm⁻³ CH3CH2CH2COO⁻Na⁺ /cm³ | volume of distilled water /cm³ | |---|---|---|---|---| | 0.050 | 25.0 | 0.005 | | | | 0.050 | 25.0 | 0.008 | | | | 0.050 | 25.0 | 0.010 | | | | 0.050 | 25.0 | 0.025 | | | | 0.030 | 15.0 | 0.050 | | | | 0.010 | 5.0 | 0.050 | | | | 0.006 | 3.0 | 0.050 | | | | 0.004 | 2.0 | 0.050 | | |[2]
5. (c)(i)Complete the table. Give your values of –log [acid]/[base] to two decimal places. | moles of acid, CH3CH2CH2COOH | moles of base, CH3CH2CH2COO⁻ | ratio of [acid]/[base] | –log [acid]/[base] | pH | |---|---|---|---|---| | 0.050 | 0.005 | 10.00 | | 3.82 | | 0.050 | 0.008 | 6.25 | | 4.01 | | 0.050 | 0.010 | 5.00 | | 4.12 | | 0.050 | 0.025 | 2.00 | | 4.70 | | 0.030 | 0.050 | 0.60 | | 5.04 | | 0.010 | 0.050 | 0.20 | | 5.52 | | 0.006 | 0.050 | 0.12 | | 5.74 | | 0.004 | 0.050 | 0.08 | | 5.91 |[1]
6. (c)(ii)Plot a graph of pH (y-axis) against –log [acid]/[base] (x-axis) on the grid on page 9. Use a cross (x) to plot each data point. Draw a line of best fit.[2]
7. (c)(iii)Circle the point on the graph you consider to be most anomalous. Suggest one reason why this anomaly may have occurred during this experimental procedure. Assume no error was made in recording the pH.[2]
8. (c)(iv)When the concentration of acid is equal to the concentration of base, pH = pKₐ. Use this information and your graph to find the value for the pKₐ. pKa = ..............................................[1]
9. (c)(v)Use your answer to (c)(iv) to calculate the value of Kₐ. State the units of Kₐ. Give your answer to three significant figures. pKa = -log Ka Kₐ = .............................................. units = ............................................[2]
10. (d)The value of pKₐ is lower when the experiment is repeated at a higher temperature. What does this tell you about the enthalpy of dissociation of CH3CH2CH2COOH? Explain your answer.[1]

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