Practical skills assessed in a written examination

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Question 31

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.

[2]

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.

[3]

Practical skills assessed in a written examination

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Question 31

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.

[2]

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.

[3]

Practical skills assessed in a written examination

Question 31

Practical skills assessed in a written examination Questions

Practical skills assessed in a written examination

Question 31

Practical skills assessed in a written examination Questions