Experimental skills

The topics covered in this Starter for ten activity are: equipment, treatment of errors, titrations, observation exercises, inferences and managing risk.

Example questions

You have just started in a new research lab and have the following equipment available to you. Choose which piece(s) of equipment you would use for each of the following tasks and name each piece you use. You can assume you have plenty of bungs, tubing and adaptors plus access to a safe source of heat;

1. To measure the volume of carbon dioxide produced by the following reaction;

ethanoic acid + sodium hydrogen carbonate → sodium ethanoate + water + carbon dioxide

1. To obtain pure water from a sample of salt water
2. To oxidise a sample of propan-1-ol to propanoic acid by heating with an excess of an acidified aqueous solution of sodium dichromate under reflux conditions
3. To make up an accurate 0.05 mol dm^–3 solution of sodium hydroxide
4. To recrystallise a sample of paracetamol contaminated with an insoluble solid impurity and collect the solid product

A student submitted the following report on his recent experiment to determine the concentration of a solution of hydrogen peroxide;

Method

10 cm³ of the hydrogen peroxide was measured using a 100 cm³ measuring cylinder and placed in a 100 cm³ conical flask.

A bung and tubing was attached and the other end connected to a 100 cm³ gas syringe. 0.2 g of the MnO₂ catalyst was weighed out.

The catalyst was added to the hydrogen peroxide solution and the bung quickly replaced.

The total volume of oxygen produced was measured.

Results

Experiment	1	2	3
Mass of MnO2 / g	0.21	0.24	0.20
Volume of gas produced / cm3	15	13	13

Analysis

The equation for the reaction is: 2 H₂O₂ → 2 H₂O + O₂

Average volume of gas produced = 13.7 cm³

Assuming room temperature and pressure, no. of moles in 13.7 cm³ of gas = 13.7 cm³ ¸ 24,000 cm³ mol^–1 = 5.694 x 10^-4 moles

2 moles of H₂O₂ produce 1 mole of O₂ so the no. of moles of H₂O₂ in 10 cm³ = 1.139 x 10^–3 moles

\concentration of H₂O₂ solution = 1.139 x 10^-3 mol ¸ 0.010 dm³ = 0.1139 mol dm^–3

1. The accuracy of each piece of equipment used is shown below.

(a) Using the individual equipment errors listed below, calculate the total percentage error for the reaction.

100 cm3 measuring cylinder ± 0.5 cm3	100 cm3 gas syringe ± 0.5 cm3
100 cm3 conical flask ± 5 cm3	Top pan balance ± 0.005 g

(b) Suggest how this percentage error could have been reduced using the same equipment.

2. The teacher wants to give the student some feedback on how he could have improved the accuracy of his experiment. Suggest two pieces of feedback she could give him.

Comment on the student’s use of significant figures in his analysis. From your comments, correct the student’s final concentration of the hydrogen peroxide.

Notes

A full version of this question and answer sheet is available from the ‘Downloads’ section below. An editable version is also available.