White vinegar is a colourless solution containing ethanoic acid. A student titrates 0.400 mol/dm³ aqueous sodium hydroxide with two samples of vinegar, A and B. The student does four titrations, two with A and two with B. The student: Step 1. uses a volumetric pipette to add 25.0 cm³ of 0.400 mol/dm³ sodium hydroxide to a conical flask Step 2. adds five drops of an indicator to the conical flask Step 3. adds A from a burette while swirling the contents of the flask, adding drop by drop near the end-point, until the solution just changes colour Step 4. empties the conical flask and rinses it with distilled water Step 5. repeats steps 1 to 4 Step 6. repeats steps 1 to 5 using B instead of A.

1. (a)(i)Fig. 2.1 shows the burette readings for the two titrations with vinegar A. [Figure 2.1] Record the burette readings in Table 2.1. Complete Table 2.1 with the volume used in each titration. Table 2.1 Titration 1 Titration 2 final burette reading/cm³ initial burette reading/cm³ volume of A used/cm³[3]
2. (a)(ii)Fig. 2.2 shows the burette readings for one of the titrations with vinegar B. [Figure 2.2] Record the burette readings for Titration 4 in Table 2.2. Calculate the volume of vinegar B added in Titrations 3 and 4. Table 2.2 Titration 3 Titration 4 final burette reading/cm³ 12.6 initial burette reading/cm³ 0.1 volume of B used/cm³[1]
3. (b)Calculate the mean volume of A and of B needed to neutralise 25.0 cm³ of 0.400 mol/dm³ aqueous sodium hydroxide. mean volume of A ..................................................... cm³ mean volume of B ..................................................... cm³[1]
4. (c)Suggest why the titrations using A and B are repeated.[1]
5. (d)The equation for the reaction between ethanoic acid and sodium hydroxide is shown. CH₃COOH + NaOH → CH₃COONa + H₂O The answer to (b) shows the mean volume of B used to neutralise 25.0 cm³ of 0.400 mol/dm³ aqueous sodium hydroxide. Calculate the concentration of ethanoic acid in B. Give your answer to an appropriate number of significant figures. concentration ..................................................... mol/dm³[2]
6. (e)Use your answer to (d) to calculate the mass of ethanoic acid in 500 cm³ of B. [M₁: ethanoic acid, 60] mass ..................................................... g[2]
7. (f)Use your answers to (b) and (e) to calculate the mass of ethanoic acid in 500 cm³ of A. mass ..................................................... g[1]
8. (g)Suggest why the conical flask is rinsed with distilled water and not 0.400 mol/dm³ aqueous sodium hydroxide between titrations.[2]
9. (h)Some vinegars are brown rather than colourless. Suggest why this titration method would not be suitable for finding the concentration of ethanoic acid in brown vinegar.[1]

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