One gram is equal to exactly 0.03527396195 ounces, so 500 grams is equal to exactly 16.763648 ounces. For practical purposes a figure of 1 gram to 0.035 is used, which would make 500 grams equal to 17.5 ounces.
Many recipes use an approximate conversion factor to simplify the recipe. Therefore, 1 ounce becomes 25 grams, not the more precise figure of 28.5, and 2.5 ounces becomes 62 grams, not 71.4. One pound of nails, for example, is likely to be sold as 500 grams, or 1/2 kilo, and not as 457 grams, which would be a more exact conversion figure.