Marking criteria for options 1 and 2:

- Correct Kc expression (formulae in square brackets).
- Substitution of equilibrium concentrations into Kc expression.
- Substitution of Kc value.
- Multiply equilibrium concentrations of I
_{2}and I by 12,3 dm^{3}. (For option 1). - Multiply the equilibrium concentrations of I by 12,3 dm
^{3}and divide the equilibrium mol of I_{2}by 12,3 dm^{3}. (For option 2). - Change in n(I) = n(I at equilibrium).
- Using ratio: I
_{2}: I = 1: 2. - Initial n(I
_{2}) = equilibrium n(I_{2}) + change in n(I_{2}). - Substitute 254g·mol
^{-1}as molar mass for I_{2}. - Final answer: (26g - 27,94g).

Marking criteria for option 3:

- Correct Kc expression (formulae in square brackets).
- Substitution of equilibrium concentrations into Kc expression.
- Substitution of Kc value.
- Change in n(I) = n(I at equilibrium).
- Using ratio: I2: I = 1:2.
- Initial [I
_{2}] = equilibrium [I_{2}] + change in [I_{2}]. - Divide by 12,3 dm
^{3}. - Substitute 254 g·mol
^{-1}as molar mass for I_{2}. - Final answer 26,543g.

**Or**

**Option 2**

I_{2} |
I | |

Initial amount (moles) | x | 0 |

Change in amount (moles) | 0,0295 | 0,0589 |

Equilibrium amount (moles) | x - 0,0295 | 0,0589 |

Equilibrium concentration (mol∙dm^{-3}) |
x - 0,0295 / 12.3 | 4,79 x 10^{-3} |

Ratio: x 12,3 and divide by 12,3.

**Or**

**Option 3**

I_{2} |
I | |

Initial concentration (mol∙dm^{-3}) |
8,497x10^{-3} |
0 |

Change (mol∙dm^{-3}) |
2,395x10^{-3} |
4,79x10^{-3} |

Equilibrium concentration (mol∙dm^{-3}) |
6,102x10^{-3} |
4,79x10^{-3} |

Using the correct ratio.