Review A-Level Edexcel Chemistry compound preparation through crystallisation, excess reagent choices, hydration errors and oxygen exclusion.
A student carried out an experiment to identify the metal M in the hydrated carbonate M2CO3⋅10H2O. A solution was made by dissolving 3.56 g of the hydrated metal carbonate in distilled water and making the volume up to 250.0 cm3 in a volumetric flask. 25.0 cm3 of this solution was placed in a conical flask and titrated with 0.100 moldm−3 of hydrochloric acid. The equation for the reaction is
The titration was repeated without using an indicator. Describe how you would obtain large, dry crystals of the metal chloride, MCl , from this titration solution. Diagrams are not required.
A description that makes reference to the following points: - evaporate/heat and to crystallisation/saturation (point) - leave to (stand and) crystallise / cool - (decant the supernatant liquid and) dry crystals between filter paper (1) Penalise use of indicator once only Penalise use of drying agent in M1 and M3 once only Ignore reference to filtration throughout Allow evaporate some/half/most of the water Allow heat until solid starts to form Allow heat to concentrate the solution Do not award evaporate to dryness Do not award heat to constant mass Do not award remove some water with drying agent Allow dry between tissue / in a desiccator/oven Ignore reference to washing of crystals prior to drying Do not award dry crystals with drying agent