Answer at least two questions from this section.See page 1 for full instructions. 1. To determine the concentration of ethanoic acid in a sample of vinegar, 25.0cm^3 of the vinegar were diluted to 250cm^3 and then the diluted vinegar was titrated with a previously standardised solution which contained 1.20 g of sodium hydroxide in 500cm^3 of solution. On average, 18.75cm^3 of the diluted vinegar were required to neutralise 25.0cm^3 of this sodium hydroxide solution. The equation for the titration reaction is: (a) Describe in detail the procedures involved in measuring a 25.0cm^3 sample of the vinegar and diluting it using deionised water to exactly 250cm^3 (15) (b) Name a suitable indicator for this titration.- Phenolohthal ein What colour change is observed at the end point? pink to clear (c) Calculate (i) the number of moles of sodium hydroxide in each 25.0cm^3 portion, (ii) the number of moles of ethanoic acid per cm^3 of diluted vinegar. (d) Find the concentration of ethanoic acid in the original vinegar (i) in terms of moles per litre, (ii) as a percentage (w/v) (e) Starting with a primary standard solution made from anhydrous sodium carbonate, what two titrations are required to standardise a sodium hydroxide solution? (9) (12 c

Question

Answer at least two questions from this section.See page 1 for full instructions. 1. To determine the concentration of ethanoic acid in a sample of vinegar, 25.0cm^3 of the vinegar were diluted to 250cm^3 and then the diluted vinegar was titrated with a previously standardised solution which contained 1.20 g of sodium hydroxide in 500cm^3 of solution. On average, 18.75cm^3 of the diluted vinegar were required to neutralise 25.0cm^3 of this sodium hydroxide solution. The equation for the titration reaction is: (a) Describe in detail the procedures involved in measuring a 25.0cm^3 sample of the vinegar and diluting it using deionised water to exactly 250cm^3 (15) (b) Name a suitable indicator for this titration.- Phenolohthal ein What colour change is observed at the end point? pink to clear (c) Calculate (i) the number of moles of sodium hydroxide in each 25.0cm^3 portion, (ii) the number of moles of ethanoic acid per cm^3 of diluted vinegar. (d) Find the concentration of ethanoic acid in the original vinegar (i) in terms of moles per litre, (ii) as a percentage (w/v) (e) Starting with a primary standard solution made from anhydrous sodium carbonate, what two titrations are required to standardise a sodium hydroxide solution? (9) (12 c

Answer 1

1. (a) (b) Phenolphthalein. Pink to clear.(c) (i) 0.0015 mol(ii)

(d) (i) 0.8 mol/L(ii) 4.804%(e) Sodium carbonate solution with hydrochloric acid; hydrochloric acid with sodium hydroxide.

Answer 2

1. (a) To measure a

sample of vinegar and dilute it to

: - Use a pipette to accurately measure

of vinegar. - Transfer the vinegar to a

volumetric flask. - Add deionised water to the flask gradually, swirling to mix. - Continue adding deionised water until the bottom of the meniscus is at the

mark. - Stopper the flask and invert it several times to ensure thorough mixing.2. (b) A suitable indicator for this titration is phenolphthalein. The colour change observed at the end point is from pink to clear.3. (c) (i) To calculate the number of moles of sodium hydroxide in each

portion: - The molar mass of sodium hydroxide (NaOH) is

. -

of NaOH is dissolved in

of solution. - Concentration of NaOH solution:

. - Moles of NaOH in

:

. (ii) To calculate the number of moles of ethanoic acid per

of diluted vinegar: - The reaction ratio between CH₃COOH and NaOH is 1:1. - Moles of ethanoic acid in

of diluted vinegar:

. - Moles of ethanoic acid per

of diluted vinegar:

.4. (d) (i) To find the concentration of ethanoic acid in the original vinegar in terms of moles per litre: - The diluted vinegar is

times less concentrated than the original. - Concentration of ethanoic acid in the original vinegar:

. - Convert to moles per litre:

. (ii) To find the concentration of ethanoic acid as a percentage (

): - Molar mass of ethanoic acid (CH₃COOH):

. - Concentration in

:

. - Convert to percentage (

):

.5. (e) To standardise a sodium hydroxide solution starting with a primary standard solution made from anhydrous sodium carbonate: - First titration: Sodium carbonate solution with a standardised hydrochloric acid solution. - Second titration: The standardised hydrochloric acid solution with the sodium hydroxide solution to be standardised.

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