chem
posted by hillary .
gold hasa density of 19 g/cm^3 if an 18 inch necklace is made of pure gold how much would it weigh in grams if it is 1 cm wide and 1 mm thick

Compute the volume in cubic centimeters.
V = (18*2.54)*1.0*(0.1) = 4.57 cm^3
Then multiply that by the 19 g/cm^3 density for the mass in grams. It is about 8.69 g. That is about 1/4 Troy oz.
Most gold necklaces are not rectangularprism blocks; they are chains, with void spaces to allow greater flexibility. The calculation indicated above is an approximation.