He got number 15 wrong… spoiler below!

Arm-Chair-Guy says this:
Answer 15.
a) Water is a molecule composed of two hydrogen atoms and one oxygen atom.
b) Every observation or examination by microscope has confirmed this.

Conclusion
Therefore we can predict that every future examination of water will reveal the same chemical composition.

Invalid. The syllogism itself is invalid and can only be considered as a strong inductive argument and not deductive. It is the ‘we can predict’ part of the conclusion which comes from nowhere, so the argument is not strictly formal. There is an ongoing debate within philosophical circles as to whether water must conform to the molecule H2O. Some argue that it is logically possible for a substance to appear exactly as water and yet still be of a different chemical composition.

I say this:

Whatever! The conclusion is proven true by contradiction. If some future observation shows a water molecule that is not H2O then premise A is wrong, but premise A is never wrong for the purposes of deciding if the conclusion is valid. QED

I think the brain starts to fry after a while :)