I often see these two words interchanged.

Problem:
The nouns “capital” and “capitol” are not synonyms.

Explanation:
The noun “capital” is from the Latin word “caput” and means head of a column or pillar.

The noun “capitol” is from the highest hill in Rome: Capitoline Hill.

The similarity between these two nouns is completely coincidental, according to some linguists.

Solution:
Use the noun “capital” to refer to a city that is an official seat of government or to refer to the wealth of an individual, group, or business. Use the noun “capitol” to refer to a building, or complex of buildings, occupied by a legislature. (Use “Capitol” to refer to the architectural structure in Washington, DC, where the United States Congress meets.)