A student wanted to find the temperature at which a known mass of potassium chloride just dissolves in water. The student's method is written below. Step 1 ...

This question is about spotting weaknesses in a solubility method and then designing a better one. The student is trying to find the temperature at which a fixed mass of potassium chloride just dissolves in a fixed volume of water.

(a) Two faults and their corrections [4]. One mark for each fault correctly identified and one for each matching correction (any two of the following pairs, maximum 4):

• The balance in Step 1 reads only to the nearest gram, which is too coarse for 5.0 g; use the balance that reads to 0.01 g [1 + 1].
• The water is "about 25 cm³", so the volume is not fixed; measure it accurately with a measuring cylinder or a pipette [1 + 1].
• The mixture is not stirred, so dissolving is slow and uneven; stir continuously with the stirring rod [1 + 1].
• The end point "most of the solid has gone" is vague; record the temperature at which the last crystal disappears [1 + 1].
• The wet thermometer carries water into the next run, changing the volume; rinse and dry it between runs [1 + 1].

(b) Carrying out the corrected experiment [3]. Record the mass to 0.01 g and close to 5.00 g [1]; record the volume of water as 25.0 cm³ [1]; and record the dissolving temperature to the nearest 0.5 °C or better [1] (for example 5.00 g, 25.0 cm³, dissolving at 34.5 °C).

(c) Apparatus for 25.0 cm³ [1]. A (volumetric or graduated) pipette [1]; a burette is also accepted. Either measures a volume to a tenth of a cm³, far more precisely than a measuring cylinder.

(d) Checking repeatability [2]. Cool the mixture until the solid just crystallises again, then reheat and take the dissolving temperature a second and a third time [1]; the values should agree to within about 1 °C, and a mean is then taken [1].

(e) Finding the mass dissolved in 25.0 cm³ of saturated solution [4].

1. Use a warmed pipette to draw off a measured volume of the clear saturated solution, leaving the undissolved solid behind [1].
2. Weigh an empty evaporating basin, then weigh it again with the solution in it [1].
3. Evaporate the water over a water bath until the mass no longer changes [1].
4. The mass of dissolved solid is the final mass minus the mass of the empty basin, scaled to 25.0 cm³ [1].

(f) Why stir throughout [1]. Stirring keeps the temperature the same throughout the mixture and brings the solid into contact with fresh solvent, which gives a sharp, reliable end point [1].

(g) Hazard and precaution [1]. Any one: the hot water and hot glassware can scald, so handle the tube with a test-tube holder or tongs and wear eye protection [1].