Final answer:
Acid X, with a pKa of 4.2, is the strongest acid among the given options because a lower pKa value indicates a stronger acid.
Step-by-step explanation:
To determine which acid is the strongest among acids X, Y, and Z, with pKa values of 4.2, 9.8, and 6.6 respectively, we need to consider the acid strength as related to pKa values. The pKa value is an inverse measure of acid strength, which means that the lower the pKa value, the stronger the acid is. Therefore, among the given acids, acid X with a pKa of 4.2 is the strongest acid because it has the lowest pKa value.
It's important to note the trends associated with acidity in relation to molecular structure factors. As an example, the electronegativity of the atom carrying the acidic proton and the size of the atom within the same period can both affect acid strength. Acids with higher electronegativity in the atom carrying the proton or larger atom size in the same period generally display stronger acid characteristics.