Question

There is a difference between your theoretical and actual volume of hydrogen gas. Assuming your technique was good and there was no human error, explain TWO possible reasons for the difference. The H2 was formed by combining HCl and Aluminum from a Coca Cola can (that was not pure aluminum),

Answer 1

When a chemical reaction occurs and hydrogen is liberated, most of it combine in pairs of atoms to form diatomic molecules (H2).
Atomic hydrogen (or nascent hydrogen) consists of individual hydrogen atoms that are not bound together like those in ordinary hydrogen molecules.
Since equal volumes of all gases contain the same number of MOLECULES, 1 litre of diatomic hydrogen has twice as much ATOMS as 1 litre of nascent hydrogen.
Since you have no control on percentage of nascent hydrogen, the final volume may vary, though the total mass is the same!