## Friday, 5 January 2018

### Pragya tested the solubility of three different substances at different temperatures and collected the data as given below Question 3 from Chapter 2. IS Matter Around Us Pure ?

(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 283   293    313    333    353 Solubility Potassium nitrate Sodium chloride Potassium chloride Ammonium chloride 21 36 35 24 32 36 35 37 62 36 40 41 106 37 46 55 167 37 54 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?

Answer :  Mass of potassium nitrate at 313 K per 100 grams of water in given saturated solution of potassium nitrate = 62 Grams
Required mass of potassium nitrate at 313 K per 50 grams of water for a saturated solution of potassium nitrate = 62 / 100 ×50 = 31 grams

(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.
Answer :  A saturated solution of potassium chloride in water at 353 K has a specific solubility.If it is allowed to cool down at room temperature around 293 K, with the decrease in temperature, solubility  of potassium chloride in the solution will also become low. It will dissolve less in water and undissolved potassium chloride result in  formation of  a layer of potassium chloride crystals at the bottom.

(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?

Answer : The  solubility of a saturated solution for a given temperature is expressed as  Mass of solute in gm per 100 gm of solvent
1. The solubility of Potassium nitrate in a saturated solution with water at 293 K = 32 gm of Potassium nitrate per 100 gm of water
2. The solubility of Sodium chloride in a saturated solution with water at 293 K = 36 gm of Sodium chloride per 100 gm of water
3. The solubility of Potassium chloride in a saturated solution with water at 293 K = 37 gm Potassium chloride per 100 gm of water
4. The solubility of Ammonium chloride in a saturated solution with water at 293 K = 35 gm Ammonium chloride per 100 gm of water
Ammonium chloride has  the highest solubility at the given temperature of 293 K

(d) What is the effect of change of temperature on the solubility of a salt?
Answer : Solubility of salt gets  gets affected with change of temperature. It increases with rise in temperature and decreases with fall in temperature.