1 mole of a substance contains Avagadro number (6.023 * 102) of molecules of that substance.
For example one mole of Carbon Monoxide (CO)
contains Avagadro number (6.023 * 1023)
molecules of CO.
We also know that
1 mole of a substance weighs Gram molecular weight of the substance.
ie one mole of CO weighs 28.01 grams Therefore 6.023 * 102 molecules of CO weighs 28.01 grams
2.57 * 1015 molecules of CO weighs? grams
2.57 * 1015
=
6.023 * 1023 * 28.01g
: 11.9581 * 10-8g
Weight of 2.57*10^{15} molecules of CO is = 11.9581 * 10^-8 g
Hope this helps