Was curious about this myself a while back and this is what I found. As per Google:
Any combination of 90-percent silver U.S. coins which have a face value of US$1.00 contains 0.715 troy ounces of 99.9-percent silver (0.7234 troy ounces if uncirculated), except for the silver dollars (Morgan and Peace) which contain .7736 troy ounces of silver.
In other words, a full troy ounce of 99.9-percent silver is contained in any combination of 90-percent silver U.S. coins which have a face value of US$1.40.
90% coins total weight: Dime=2.5g Quarter=6.25g Half Dollar=12.5g
Makes sense.