C7H16, where C=12.01, and H=1.01, so the weight of the molecule would be 7(12.01)+16(1.01), or 100.23. The percentage of carbon would be found by ((7*12.01)/100.23)*100=83.88% Carbon
((16*1.01)/100.23)*100=16.12% Hydrogen
Technically molar mass cannot be in grams, it is in grams per mole. and it refers to a specific number of molecules of a substance, therefore substances have different molar masses because the elements have different weights. for example having 10 water molecules would be a lot heavier than having 10 air molecules
He could be blindfolded and know which was his and which was his sister's. All he would need to do is pick them both up and if they were too big then pick them up one at a time. The lumber might make it harder to tell, but this is a question about physical properties.
So there is a change in mass which for the purpose of this question should be quite different. His sister's ought to be much lighter than his. He would find it easier to pick up.
