Experimental work Question 7

Paper 3 Section A Experimental work Question 7

 Teacher only box

This page of questions on experimental work in preparation for Section A of Paper 3 can be marked as direct student access either for assigning as a test or for students to work on in their own time. If you do not wish to use student access, links to downloadable versions of the questions and, separately the worked answers, can be found at Printable versions of written tasks.

 

 

An experiment was performed to find the concentration of a solution of sodium hydroxide.

1.203 g of hydrated oxalic acid crystals, (COOH)2.2H2O, were dissolved in distilled water and the total volume made up to 100 cm3. This solution was used to fill a burette.

10.0 cm3 of sodium hydroxide solution of unknown concentration were added to a conical flask and titrated with the oxalic acid solution. Phenolphthalein was used as the indicator.

The titration was repeated two more times to determine the average volume of the oxalic acid solution required to reach the end point.

(a) A student calculated the concentration of the oxalic acid solution to be 0.134 mol dm−3. He arrived at this answer by multiplying the mass taken by ten to give the mass in 1.00 dm3 of solution and then dividing by the molar mass of oxalic acid.

(10 x 1.203 ) ÷ ((2 x 12.01) + (4 x 16.00) + (2 x 1.01)) = 0.134 mol dm−3.

Outline the major error in the student’s calculation and determine the correct value for the concentration. [2]

3 lines
Show model answer

(b) Identify the piece of glassware that should be used to ensure the total volume of 100 cm3 is as accurate as possible when preparing a standard solution. [1]

1 line
Show model answer

(c) Before placing the oxalic acid solution in the burette the student washed out the burette with distilled water but then omitted to dry it before adding the oxalic acid solution. Explain how this would affect the final result for the concentration of the sodium hydroxide solution. [1]

2 lines
Show model answer

(d) The student also washed out the conical flask with distilled water and omitted to dry it before adding the 10.0 cm3 of the sodium hydroxide solution and then performing the titration with the oxalic acid solution. Explain the effect this would have on the final result for the concentration of the sodium hydroxide solution. [1]

2 lines
Show model answer

(e) Oxalic acid is a diprotic acid. Assuming the sodium hydroxide solution has a very similar concentration to the solution of oxalic that was prepared, predict how the electrical conductivities of the two solutions will compare. [1]

3 lines
Show model answer

 

 

All materials on this website are for the exclusive use of teachers and students at subscribing schools for the period of their subscription. Any unauthorised copying or posting of materials on other websites is an infringement of our copyright and could result in your account being blocked and legal action being taken against you.