Question 3 Pragya tested the solubility of three different substances at different temperatures and collected the data as given below( results are given in the following table, as grams of substance dissolved in 100 grams of water to form a saturated solution). Substance dissolved Temperature in K Temperature in K Temperature in K Temperature in K Temperature in K Substance dissolved 283 293 313 333 353 Potassium nitrate 21 32 62 106 167 Sodium chloride 36 36 36 37 37 Potassium chloride 35 35 40 46 54 Ammonium chloride 24 37 41 55 66 (a) What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 grams of water at 313 K? (b) Pragya makes a saturated solution of potassium chloride in water at 353 K and leaves the solution to cool at room temperature. What would she observe as the solution cools? Explain. (c) Find the solubility of each salt at 293 K. What salt has the highest solubility at this temperature? (d) What is the effect of change of temperature on the solubility of a salt?
Class 9 - Science - Is Matter Around Us Pure Page 28
Answers
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>> (a) The solubility of potassium nitrate at 313 K is 62 grams ( See the given table ). This means that 62 grams of potassium nitrate is needed to make a saturated solution of potassium nitrate in 100 grams of water at 313 K, So, to make a saturated solution in 50 grams of water we will need half of 62 grams of potassium nitrate, which is 62/2= 31 grams of potassium nitrate.
(b) When a saturated solution of potassium chloride at 353 K is left to cool, then solid potassium chloride ( or crystals of potassium chloride ) will gradually separate out from the solution (because the solubility decreases on cooling).
(c) The solubility of various salts at 293 K is : Potassium nitrate 32 g ; Sodium chloride 36 g ; Potassium chloride 35 g ; and Ammonium chloride 37 g. Ammonium chloride has the highest solubility (of 37 g) at this temperature of 293 K.
(d) The given data shows that the solubility of a salt increases on increasing the temperature.
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Answer:
(a) At 313 K 62 g of potassium nitrate dissolved in 100 g of water. So to produce a saturated solution of potassium nitrate in 50 g of water we need
100
62
×50=31 g of potassium nitrate.
(b)Some soluble potassium chloride will separate out in the form of crystal at room temperature because the solubility of potassium chloride will decrease.
(c)
(i) Solubility of Potassium nitrate at 293 K is 32 g.
(ii) Solubility of sodium chloride at 293 K is 36 g.
(iii) Solubility of Potassium chloride at 293 K is 35 g.
(iv) Solubility of Ammonium chloride at 293 K is 37 g.
The solubility of Ammonium chloride is highest at this temperature.
(d)The solubility of salt increases with the increase in temperature.