quarters is / are

There is / are quarters of the alcohol left.
Strange sentence. Instead:
There are three quarters of the alcohol left.

Three quarters of the alcohol is / are left. Are.

I'd say is.

There is three quarters of the alcohol left. It's a single measurement. Like saying there is two inches of beer left at the bottom of the barrel.

I'd say is.

There is three quarters of the alcohol left. It's a single measurement of the total.

Suppose we had 4 pints of beer, each pint in a separate glass. Bill drank one.

There are 3 pints of beer left.

Three-quarters of the beer (substance - non-count) is still sitting on the table.

Three quarters of the beers (servings of beer -count) are stil

Nona seems to be right.

At Yahoo:

When the object is singular (in your case, "of the alcohol"):
1,100 for "three quarters of it is"
72 for "three quarters of it are"

But in the case of the plural, are seems to be preferable:
5,640 hits for "three quarters of them