A Whiskey Question for Chemists

Here’s the scenario: You fill a cask (let’s say a barrel or 31 gallons) of fresh distillate that is sixty percent alcohol by volume. The other forty percent is water.

Every year, two percent of the contents of the barrel evaporates.

Eight years later, you tap the cask and bottle. The question I have is thus: What’s the Alcohol content of the spirit?

The reason I ask (and I admit, I should really know this by now), is that I cannot get anyone in the industry to give me a straight answer. I had an heir apparent to a whiskey empire tell me it’s mostly the alcohol that evaporates. Elsewhere, I had a marketing director tell me it was the water that evaporates. The books I have read have given me either vague or contradictory answers. And the two times I had the opportunity to ask a master distiller, I forgot completely to bring it up.

As I wrote in the Heritage Bourbon post, my belief is that both alcohol and water will evaporate, but the ration depends on several variables, like heat, humidity, etc. etc. Am I close?

Is there a rule or law that will help me understand just what the heck is going on?