CHM 2046C Module 10 Sample Exam Answer

Part F. Calculations based on K_c 10 points

(Calculation Equilibrium Concentrations from Initial Concentrations)

At 25^oC, K_c=0.090 for the reaction:

H₂O (g) + Cl₂O (g) ç è 2 HOCl (g)

Calculate the concentration of all species if 2.0 mole of pure H₂O and 2.0 mole of pure Cl₂O are placed in a 2.0 L flask and the system is allowed to come to equilibrium.

McMurry 5^th: Reference Section 13.5 pages 511-516. Look at worked example 13.9 page 513 and 13.10 page 514. Read the steps in Figure 13.6:

Step 2:

	[H2O (g)]	[Cl2O (g)]	[HOCl (g)]
Initial M	2.0 mol/2.0 L= 1.0M	1.0M	0
Change	-x	-x	+2x
Equilibrium M	1.0 - x	1.0 - x	2x

[HOCl]²

K_c = ---------------------

[H₂O ]¹ [Cl₂O]¹

Step 3:

[2x]²

0.090 = ---------------------

[1.0-x ]¹ [1.0-x]¹

[2x]²

0.090 = --------------

[1.0-x ]²

Take the square root of both sides:

[2x]

(0.090)^½ = --------------

[1.0-x ]

0.30 (1.0 – x) = 2x

0.30 – 0.30x = 2x

0.30 = 2.30x

x = 0.13

Step 4:

Substituting for x:

	[H2O (g)]	[Cl2O (g)]	[HOCl (g)]
Initial M	2.0 mol/2.0 L= 1.0M	1.0M	0
Change	-.13	-.13	+.26
Equilibrium M	0.87	0.87	0.26

Additional Reference

Kotz 6^th Reference section 16.4 p 772-777 See Example 16.5 p771 using K_c without a quadratic equation to solve and 16.6 using K_c and needing the quadratic expression. Try Exercise 16.6 p774 for the first type and Exercise 16.7 p777 for the second type; then go to the end of the chapter on page 790-791 and try #13-#18. The Solutions manual for these exercises have been distributed.