6a. When the ionic solid (NH4)2SO4 dissolves in water the temperature of the sol

Question

6a. When the ionic solid (NH4)2SO4 dissolves in water the temperature of the solution decreases. In terms of the steps of the solution process explain why (NH4)2SO4 dissolving in water is endothermic.

b) You are responsible for preparing 250.00 mLs of a 0.450 M (NH4)2SO4 solution using pure (NH4)2SO4, distilled water and any equipment you require. Explain how you would prepare this solution. (show important calculations)

c) The density of the solution in b) is 1.036 g cm-3. Calculate the weight percent of the solute in the solution.

d) Calculate the ideal freezing point of the solution in b).  

e) A student carefully determines the experimental freezing point of this solution. She finds the freezing point is –2.02 C. How would you explain the difference between the experimental and the ideal freezing point of this solution?

f)You are given a compound with the formula CH4N2. The experimental freezing point of a solution prepared by dissolving 4.40 g of CH4N2 in 100. grams of water was found to be –0.26 C.(9) i) Is the compound ionic or covalent? Explain.

ii) Re-write the formula to reflect the type of compound you indicated in i).

Explanation / Answer

a) The steps involved in the dissolution of a solid ionic compound in water can be enlisted as below.

i) The solvent (water) molecules move apart from each other, making way for the solid solute or ions. This process requires a weakening of the intermolecular forces in water (mainly hydrogen bonding) and is an energy-demanding process. Consequently, energy must be supplied from outside and hence this step is endothermic.

ii) The ionic solid, (NH4)2SO4 dissociates into positive and negative ions once introduced in water. The oppositely charged ions are held together by attractive forces. Ion pairs are arranged in definite geometric patterns and the energy required to break these ion pairs is the lattice disruption energy of the solid. Once the ion pairs are separated, the individual ions are separated by supplying energy. Therefore, both the processes listed above are endothermic.

iii) The oppositely charged ions are surrounded by water molecules via ion-dipole interactions. This process releases energy and hence, is exothermic.

In case of dissolution of solid ionic (NH4)2SO4, the energy required or supplied (sum of the contributions in (i) and (ii) above) outweigh the energy released (iii) and hence, the dissolution process is endothermic.

b) Moles of (NH4)2SO4 in 250.00 mL of 0.450 M solution = (250.00 mL)*(1 L1/000 mL)*(0.450 M) = 0.1125 mole.

Molar mass of (NH4)2SO4 = [2*(1*14.0067 + 4*1.008) + 1*32.065 + 4*15.9994] g/mol = 132.14 g/mol.

Mass of (NH4)2SO4 corresponding to 0.1125 mole = (0.1125 mole)*(132.14 g/mol) = 14.86575 g 14.8657 g.

In order to prepare 250.00 mL of 0.450 M (NH4)2SO4, weigh out 14.8675 g solid (NH4)2SO4 in a 250.00 mL volumetric flask and top up with distilled water (ans).

c) The density of the solution in (b) is 1.063 g/cm3.

We know that 1 cm3 = 1 mL; therefore, 1 mL of the solution weighs 1.063 g.

Now, the solution is composed of solid (NH4)2SO4 and the mass of the solution = mass of solid (NH4)2SO4 + mass of water.

The density of water is 1 g/mL, i.e, 1 mL water weighs 1 g.

Therefore, mass of solid (NH4)2SO4 in 1 mL solution = (1.063 – 1) g = 0.063 g.

100 mL (= 100 g) water will dissolve (0.063 g/1 g)*(100 g) = 6.3 g solid (NH4)2SO4.

The mass percent strength of the solution is 6.3% (ans).

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