Chapter 2 Atoms, Molecules, and Ions - Study Questions - Page 95g: 117

$1.7\times10^{21}\ molecules$

Molar mass $18.015\ g/mol$ Mass of a drop: $0.050\ cm^3\times1.00\ g/cm^3=0.050\ g$ Number of moles in a drop: $0.050\ g/18.015\ g/mol = 2.8\times10^{-3}\ mol$ Number of molecules: $2.8\times10^{-3}\ mol\times6.022\times10^{23}\ molecules/mol=1.7\times10^{21}\ molecules$