Sea water contains about $$\(20 \mathrm{gdm}^{-3}\)$$ of chloride ions, $$\(\mathrm{Cl}^{-}(\mathrm{aq})\)$$. The exact concentration of $$\(\mathrm{Cl}^{-}(\mathrm{aq})\)$$ in sea water can be determined by titration with aqueous silver ions, $$\(\mathrm{Ag}^{+}(\mathrm{aq})\)$$, using aqueous potassium chromate(VI), $$\(\mathrm{K}_{2} \mathrm{CrO}_{4}(\mathrm{aq})\)$$, as an indicator. When aqueous silver nitrate, $$\(\mathrm{AgNO}_{3}(\mathrm{aq})\)$$, is added to a sample of sea water, silver ions react with chloride ions to form a precipitate of silver chloride. $$\[ \mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq}) \rightarrow \mathrm{AgCl}(\mathrm{s}) \]$$ When all of the $$\(\mathrm{Cl}^{-}(\mathrm{aq})\)$$ has reacted with $$\(\mathrm{Ag}^{+}(\mathrm{aq})\)$$, the presence of unreacted $$\(\mathrm{Ag}^{+}(\mathrm{aq})\)$$ is detected by chromate(VI) ions, $$\(\mathrm{CrO}_{4}{ }^{2-}(\mathrm{aq})\)$$. A red precipitate of $$\(\mathrm{Ag}_{2} \mathrm{CrO}_{4}(\mathrm{~s})\)$$ is seen. $$\[ 2 \mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{CrO}_{4}{ }^{2-}(\mathrm{aq}) \rightarrow \mathrm{Ag}_{2} \mathrm{CrO}_{4}(\mathrm{~s}) \]$$ The amount of $$\(\mathrm{Ag}^{+}(\mathrm{aq})\)$$ reacting with $$\(\mathrm{Cl}^{-}(\mathrm{aq})\)$$ in the sample of sea water can be calculated in order to determine the concentration of $$\(\mathrm{Cl}^{-}(\mathrm{aq})\)$$ in the sample of sea water. A student uses the following method. step 1 Use a weighing boat to weigh by difference approximately 10.6 g of $$\(\mathrm{AgNO}_{3}(\mathrm{~s})\)$$ into a $$\(100 \mathrm{~cm}^{3}\)$$ glass beaker. step 2 Use the sample of $$\(\mathrm{AgNO}_{3}(\mathrm{~s})\)$$ in the glass beaker to prepare $$\(250.0 \mathrm{~cm}^{3}\)$$ of $$\(\mathrm{AgNO}_{3}(\mathrm{aq})\)$$. step 3 Transfer this solution into a dark brown glass bottle. Label this solution $$\(\mathbf{X}\)$$. step 4 Collect a sample of sea water and remove any solid material present. step 5 Transfer $$\(10.00 \mathrm{~cm}^{3}\)$$ of the sea water into a conical flask. step 6 Add $$\(1 \mathrm{~cm}^{3}\)$$ of $$\(\mathrm{K}_{2} \mathrm{CrO}_{4}(\mathrm{aq})\)$$ to the conical flask. step 7 Rinse a burette in preparation for the titration. step 8 Fill the burette with solution $$\(\mathbf{X}\)$$. step 9 Slowly add solution $$\(\mathbf{X}\)$$ to the conical flask until the white precipitate turns red. This is the end-point. Describe how the student should prepare $$\(250.0 \mathrm{~cm}^{3}\)$$ of $$\(\mathrm{AgNO}_{3}(\mathrm{aq})\)$$ in step 2, starting with the $$\(\mathrm{AgNO}_{3}(\mathrm{~s})\)$$ in the $$\(100 \mathrm{~cm}^{3}\)$$ beaker in step 1. ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . .........................................................................................................................................

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