Everything Science Grade 12

CHAPTER 3. REACTION RATES 3.7

Calculate the value of Kc.

SOLUTION

Step 1 : Write the equation forkc

Kc=

[NO][SO 3 ]

[SO 2 ][NO 2 ]

Step 2 : Fill in the values you know for this equation and calculate Kc

Kc=

(0. 4 × 0 .2)

(0. 2 × 0 .1)

= 4

Example 3: Calculating reagent concentration

QUESTION

For the reaction:

S (s) + O 2 (g)� SO 2 (g)

1. Write an equation for the equilibrium constant.


2. Calculate the equilibrium concentration of O 2 if Kc=6 and [SO 2 ] =
   3 mol· dm−^3 at equilibrium.

SOLUTION

Step 1 : Write the equation forKc

Kc=

[SO 2 ]

[O 2 ]

(Sulphur is left out of the equation because itis a solid and its

concentration stays constant during the reaction)

Step 2 : Re-arrange the equation so that oxygen is on its own on one side

of the equation

[O 2 ] =

[SO 2 ]

Kc

Step 3 : Fill in the values you know and calculate [O 2 ]