The conductivity of an ionic solution can be determined by passing an electric current through the solution and measuring the conductivity using a conductivity meter.

1. (a)(i)A student carries out an experiment to measure the conductivity of different solutions of ethanoic acid, CH₃COOH, which is a weak acid. The acid dissociation constant, Kₐ, can be determined from this experiment. The student makes standard solution A, 250.0 cm³ of 2.00 mol dm⁻³ CH₃COOH(aq). State what is meant by a standard solution.[1]
2. (a)(ii)Describe how the student should make standard solution A from pure ethanoic acid. The concentration of standard solution A should be 2.00 mol dm⁻³ to the nearest three significant figures. Your answer should state the name and capacity of any apparatus that the student should use. A balance is not available. Pure ethanoic acid is a liquid with a density of 1.05 g cm⁻³ at room temperature. You may wish to write your answer using a series of numbered steps. [Mᵣ: CH₃COOH, 60.0][4]
3. (b)The student wears chemically resistant gloves throughout this procedure. Suggest why.[1]
4. (c)(i)The student dilutes standard solution A with distilled water to make solutions of different concentrations. The conductivity of these solutions is measured. Table 2.1 shows the results. Table 2.1 | | 1 | 2 | 3 | 4 | |---|---|---|---|---| | | [CH₃COOH] /mol dm⁻³ | conductivity /S dm⁻¹ | 1 √ [CH₃COOH] /dm¹.⁵ mol⁻⁰.⁵ | molar conductivity, Λᴍ /dm² S mol⁻¹ | | | | | 0.0500 | 3.76 × 10⁻³ | | | | 0.0250 | 2.74 × 10⁻³ | | | | 0.0125 | 1.92 × 10⁻³ | | | | 0.00625 | 1.33 × 10⁻³ | | | | 0.003125 | 1.15 × 10⁻³ | | | | 0.0015625 | 6.68 × 10⁻⁴ | | | In order to determine Kₐ the results must be used to obtain two sets of data. Column 3 is the reciprocal of the square root of the concentration (column 1). Column 4 is the molar conductivity, Λᴍ, which is found by dividing the conductivity (column 2) by the concentration (column 1). Complete columns 3 and 4 in Table 2.1. Give all values to three significant figures.[2]
5. (c)(ii)Identify the dependent variable in this experiment.[1]
6. (c)(iii)The student decided to measure the conductivity of distilled water at the start of the experiment. Suggest why.[1]
7. (c)(iv)State one variable that should be controlled.[1]
8. (d)Plot a graph on the grid to show the relationship between molar conductivity, Λᴍ, and 1 √ [CH₃COOH] . Use a cross (x) to plot each data point. Draw a line of best fit that includes the origin. [Graph Grid with labels: molar conductivity, Λᴍ /dm² S mol⁻¹ (y-axis) and 1/√[CH₃COOH] /dm¹.⁵ mol⁻⁰.⁵ (x-axis)][2]
9. (e)Circle one point on the graph that you consider to be most anomalous. The conductivity meter was correctly functioning. Suggest one reason why this anomaly may have occurred during this experimental procedure.[2]
10. (f)(i)The equation for the line of best fit is shown. ΛΜ = Λa Ka / √ [CH3COOH] Λ = 3.91 dm² S mol⁻¹ Use your graph to determine the gradient of the line of best fit. State the coordinates of both points you used in your calculation. These must be selected from your line of best fit. Give the gradient to three significant figures. coordinates 1 coordinates 2 gradient = dm⁰.⁵ S mol⁻⁰.⁵[2]
11. (f)(ii)Determine the acid dissociation constant, Kₐ, of ethanoic acid. Include units in your answer. Kₐ = units = [2]
12. (g)The student repeats the experiment with propanoic acid. The numerical value of the Kₐ of propanoic acid is experimentally determined as 1.28 × 10⁻⁵. The theoretical numerical value is 1.34 x 10⁻⁵. Comment on the validity of the experimental result. Assume the maximum total percentage error from the measurements made is 6.5%.[2]

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