The viscosity of a substance is a measure of how quickly the substance flows when it is subjected to a force such as gravity. The viscosity of a liquid or solution is dependent on: • size of molecules • strength of intermolecular forces of attraction • temperature. It is possible to calculate the mean molecular mass (mean Mr) of a polymer in solution by measuring the viscosity of solutions of the polymer at different concentrations. Measurements related to the viscosity of a solution can be made using a capillary viscometer, shown in the diagram. [Figure X.X] • The apparatus is set up as shown. • The bung is removed and the solution falls through the capillary section. • The time taken for the top of the solution to pass between the two marks at the top (A) and bottom (B) of the reservoir is recorded. • This time taken is related to the viscosity of the solution. A student plans an experiment to calculate the mean Mr of molecules of poly(phenylethene). The student plans to make solutions of different concentrations of poly(phenylethene) dissolved in methylbenzene, C6H5CH3, an organic solvent.

1. (a)Before the experiment, a mixture of concentrated nitric acid and concentrated hydrochloric acid is passed through the capillary viscometer. The capillary viscometer is then rinsed, first with water, and then with propanone. Suggest why the capillary viscometer is rinsed with water and then with propanone. rinse with water ________________________________________________________________________________________________________________ rinse with propanone ________________________________________________________________________________________________________________[2]
2. (b)A constant, η, related to the viscosity of a solution can be found by plotting a graph of (¹⁄c)log(ᵗ⁄t₀) on the vertical axis against c on the horizontal axis. c = concentration of poly(phenylethene) in C6H5CH3 (in gdm⁻³) t = time taken for the solution to pass between marks A and B (in s) t₀ = time taken for pure C6H5CH3 to pass between marks A and B (in s) The results of a series of experiments using different concentrations of poly(phenylethene) in C6H5CH3 are shown. The values of t/t₀ have been calculated for you. Process the results to complete the table. Record all your data to three significant figures. concentration of poly(phenylethene), c / g dm⁻³ | ¹⁄c / dm³g⁻¹ | time taken, t / s | t/t₀ | log(t/t₀) | (¹⁄c)log(t/t₀) / dm³g⁻¹ ---|---|---|---|---|--- 16.0 | | 176 | 2.26 | | 14.0 | | 164 | 2.10 | | 12.0 | | 151 | 1.94 | | 10.0 | | 138 | 1.77 | | 8.0 | | 125 | 1.60 | | 6.0 | | 113 | 1.45 | | 4.0 | | 102 | 1.31 | | 2.0 | | 89 | 1.14 | | [3]
3. (c)Plot a graph on the grid to show the relationship between (¹⁄c)log(ᵗ⁄t₀) and c. Use a cross (x) to plot each data point. Draw the straight line of best fit.[2]
4. (d)(i)Capillary viscometer measurements are usually made at 25°C. Predict the effect on the time taken for the solution to fall between marks A and B if a solution of temperature 18°C is tested in the viscometer. Explain your answer.[2]
5. (d)(ii)Suggest how a student could ensure that a measurement is made at 25°C.[1]
6. (e)The data you have plotted shows an anomaly that comes from the results obtained. Circle the anomalous point on the graph. Suggest a reason for this anomaly. Assume that for this result, the concentration of the solution was correct.[1]
7. (f)(i)The y-axis intercept on the graph in (c) is equal to η/2.30, where η is a constant for poly(phenylethene) at 25°C. Use the graph you plotted in (c) to find a value for η. η = __________ dm³g⁻¹[1]
8. (f)(ii)The relationship between η and the mean Mr is shown. (K and a have specific values for solutions of poly(phenylethene).) η = K × (mean Mr)ᵃ For a solution of poly(phenylethene) dissolved in C6H5CH3 the relationship can be expressed as shown. log (mean Mr) = 1.59log η + 7.03 Use your value of η calculated in (f)(i) to calculate a value for the mean Mr of poly(phenylethene) in this experiment. mean Mr = __________[2]
9. (f)(iii)Poly(phenylethene) forms when molecules of phenylethene, CH2CHC6H5, undergo addition polymerisation. xCH2CHCH5 → (CH2CHC6H5)x Use the value of mean Mr you calculated in (f)(ii) to calculate a value for x, the number of repeat units in the polymer. Your answer should give the nearest whole-number value of x. If you were unable to calculate a value in (f)(ii), then you may use mean Mr = 1.56 × 10⁵, but this may not be the correct answer. [Ar: C, 12.0; H, 1.0] x = __________[1]
10. (g)In the equation, η = K × (mean Mr)ᵃ, a depends on the strength of the intermolecular forces between the solvent and the solute. The value of a increases as the intermolecular forces between solvent and solute increase. Predict how the value of a for poly(ethenol) dissolved in water differs from a for poly(phenylethene) dissolved in C6H5CH3. Explain your answer.[2]

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