You will determine the percentage purity of another sample of calcium carbonate, $$\(\mathrm{CaCO}_{3}\)$$, by titration. The experiment involves three steps. step 1 A known mass of the same impure calcium carbonate, FA 1, is reacted with an excess of hydrochloric acid to form FA 3. This step has been done for you. step 2 You will dilute the products of step 1 to a known volume. step 3 You will carry out a titration to find out how much acid remains after the reaction in step 1. FA 3 has been prepared by reacting 19.0 g of FA 1 with $$\(250 \mathrm{~cm}^{3}\)$$ of $$\(2.00 \mathrm{~mol} \mathrm{dm}^{-3}\)$$ hydrochloric acid, HCl . FA 5 is $$\(0.0900 \mathrm{~mol} \mathrm{dm}^{-3}\)$$ sodium hydroxide, NaOH . FA 6 is bromophenol blue indicator. The percentage purity of calcium carbonate in FA 1 has been determined by two different methods, gas collection and titration. The gas collection method used in Question 1 is less accurate. (i) Suggest two reasons why the gas collection method is less accurate. 1 . 2 . (ii) Describe a change to the gas collection method that would improve the accuracy of the percentage purity determined in Question 1. . . You should record clearly at what stage in a test an observation is made． Where no change is observed，you should write＇no change＇． Where reagents are selected for use in a test，the name or correct formula of the element or compound must be given． If any solution is warmed，a boiling tube must be used．If a solid is heated，a hard－glass test－tube must be used． Rinse and reuse test－tubes and boiling tubes where possible． No additional tests should be attempted．

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