c2 making salts, mark my 6 mark question please.

Badges: 10
Report Thread starter 5 years ago
Describe a method for making crystals of potassium chloride from potassium hydroxidesolution and hydrochloric acid. In this method you should:  describe how you will add the correct amount of the hydrochloric acid to neutralisethe potassium hydroxide solution  describe how you will get crystals of potassium chloride.

ok i wrote Add potassium hydroxide to a beaker and then an indicator and the hydrochloric acid then stir and mix, then keep adding drops of volume of hydrochloric acid until the indicator turns green then note the volume of hydrochloric acid , then use charcoal to remove the indicator or repeat the steps using the volume of hydrochloric acid noted- then pour solution into a dish and evaporate the water and leave it for a few days for crystal salts to form.

Badges: 18
Report 5 years ago
• One reagent in beaker (or similar)
• Add (any named) indicator
• Add other reagent
• Swirl or mix
• Add drop wise near end point
• Stop addition at change of indicator colour
• Note volume of reagent added
• Repeat without indicator, adding same volume of reagent or remove indicator using charcoal
• Pour solution into basin / dish
• Heat (using Bunsen burner)
• Leave to crystallise / leave for water to evaporate / boil off water

This is what the mark scheme says. You have included a detailed description of a laboratory procedure for obtaining potassium chloride from potassium There is a detailed description of a laboratory procedure for obtaining potassium chloride from potassium hydroxide solution. Coherently, you have described the used of an indicator clearly but haven't stated what type of indicator. You have also described how to form the crystals.

I would, personally, award this a 4/5 as I find your method a little hard to follow. If you reworded it, you'd easily be on for 6/6. For example:

Firstly, add potassium hydroxide to a breaker with indicator. Then, add the other reagent and in this case it is hydrochloric acid. Mix the solution thoroughly. Next, keep adding drops of a volume of hydrochloric acid until the indicator turns green to indicate that it is a neutral solution. Once the indicator has turned green, you should stop adding the hydrochloric acid. Note the volume of hydrochloric acid that you used as this will be needed for the next task. Using charcoal, remove the indicator or you could repeat the experiment without adding the indicator but keeping the volume of acid the same. With this solution, carefully, pour it into a basin, or dish, and apply some heat using the Bunsen burner leaving the water to evaporate. Finally, leave it for a few days until crystal salts begin to form. You now have potassium chloride crystals. Just these extra little details like 'to make the solution neutral' proves to the examiner clearly what you're talking about. On the six mark questions, avoid writing in long winded sentences as it can become a) hard to read and b) could impede the communication of your answer.

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