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  • IMAT 2015 Chemistry Q50

  • Ari Horesh

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    June 18, 2021 at 6:39 pm
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    Potassium nitrate was found to have a solubility of 120 g in 100 g of water at 80 °C and 50 g in
    100 g of water at 25 °C.
    50 g of water was heated to 80 °C and solid potassium nitrate added until the solution was just
    saturated.
    The solution was then cooled to 25 °C when solid potassium nitrate separated out to leave a
    saturated solution.
    Using the information provided in this question, what is the minimum mass of water that must
    now be added to the mixture of the solution and the solid in order to make this solid potassium
    nitrate redissolve at 25 °C?
    20 g
    140 g
    120 g
    70 g
    190 g

  • Paris Pitsinikos

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    June 18, 2021 at 6:52 pm
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    This question is based on the supersaturated solutions and their response to temperature change. First, we know that at 80 degrees Celsius we can dissolve 120 grams of solute in 100 grams of water so in 50 grams of water we can dissolve 60 grams of solute. The same logic goes for the 25 degrees solution where we can dissolve 25g of solute in 50 grams of water. Thus, the decrease in temperature will leave us with the difference 60g – 25g = 35g being transformed into a solid. To dissolve this 35g we have to calculate: in 100g of water we can dissolve 50g of solute. How much water do we need to dissolve 35g of solute? 50x = 100 * 35 = 70g of water. The answer is ‘D’.

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