Try this solution:
Given:
N(H)=3.61*10'24;
N(Av)=6.022*10'23;
M(H₂O)=18 (gr/mol).
Calculate: m=?
1) Formula: m=M*ν, where ν=N/N(Av)=[moles], and N(Av) - Avogadro's number.
2) if N(H) is number of atoms, then number of molecules is N(H)/2 (water - H₂O, where 2 atoms of H and 1 atom of O);
3) According to the formula above:
m=M*N(H)/(2N(Av));
m=18*3.61*10'24 / (2*6.022*10'23) ≈ 53.952 (gr.)
