Since U.S. pennies weigh 2.5 grams each, it takes 40 pennies to make 100 grams. However, pennies minted before 1982 weigh 3.11 grams, so it would take only 33 of them to add up to 100 grams.
The weight discrepancy between pennies minted before and after 1982 is a result of different compositions. Pre-1982 pennies are composed of 95 percent copper, while post-1982 pennies are 97.5 percent zinc and coated with a thin layer of copper. The U.S. Mint switched to making pennies primarily out of zinc because copper had become prohibitively expensive. Both copper and zinc pennies were minted in 1982; a scale is the only way to tell the composition of a penny minted in that year.