Question

Weight of 3×10^5 molecule of H2o is

Answer 1

molar mass :- mass of the one mole of the substance.
molar mass of the H2O = 2+16 = 18g/mol

one mole of any substance contains 6.022 × 10^23 molecules.

18 gram contains = 6.022×10^23 molecules
" X " grams contains = 3×10^5 molecules
[ cross multification ]

X = 18 × 3 × 10^5/6.022×10^23

X = 54×10^5/6.022×10^23

X = 8.9671 × 10^5-23

X = 8.97 × 10^-18 g

so mass of the 3 × 10^5 molecules of H2O is 8.97 × 10^-18 grams.
__________________________________________________

onother method

one mole of any substance contains exactly avogadro number of molecules.

molecules = number of moles × 6.022 × 10^23

molecules = mass/molar mass × 6.022 × 10^23

3×10^5 = mass/18 × 6.022×10^23

3×10^5×18 = mass × 6.022×10^23

54 × 10^5 = mass × 6.022×10^23

mass = 54×10^5/ 6.022×10^23

mass = 8.97 × 10^5-23

mass = 8.97 × 10^-18 g