Question 9

A student plans to calibrate a high-precision salinity sensor by measuring the electrical conductivity of sodium chloride ( $NaCl\text{NaCl}$ ) solutions at two different temperatures, $15∘C15^\circ\text{C}$ and $25∘C25^\circ\text{C}$ .

The student wants to prepare a $dm−30.05\text{ mol dm}^{-3}$ $NaCl\text{NaCl}$ solution for each temperature trial.

The student uses the following method to prepare each $dm−30.05\text{ mol dm}^{-3}$ $NaCl\text{NaCl}$ solution:

Perform two sequential dilutions from a $dm−35.0\text{ mol dm}^{-3}$ $NaCl\text{NaCl}$ stock solution.
For each dilution, use a measuring cylinder to measure the volume of deionised water and a plastic transfer pipette to transfer the salt solutions.

Suggest two ways in which the student can reduce the percentage error in their measurements when preparing these $NaCl\text{NaCl}$ solutions.

Complete the sentences below to outline a robust experimental procedure:

To prevent pre-existing ionic impurities from altering the conductivity, the solutions must be prepared using deionised or ............................... water.
To maintain a constant temperature during the conductivity measurements, the vessel containing the solution must be placed in a temperature-controlled ............................... .
After repeating the measurement with 15 independent trials at each temperature, the student calculates the mean conductivity and the ............................... to determine the precision and spread of the data.