One pound = 453.59 grams
1 ounce = 28.35 grams
A US Silver dollar weighs 26.73 grams. Of that, 90% is silver and 10% is copper.
The exact answer is 141.75 grams equal to 5 ounces. (That is rounded to the nearest hundredth of a gram, which is close enough for most work.) If you are wanting to know this for the Cub Scout...
31.1 grams in a troy ounce of silver.