A solution is made by dissolving sodium chloride in water and its concentration is expressed as 0.9% by mass.
Calculate:
(i) the number of moles, and
(ii) number of molecules present in NaCl for this solution.
[Given: mass % = Mass of solute / Mass of solution ×100]
(Atomic mass of Na = 23.0 u, Cl = 35.5 u)
0.9% mass by mass means 0.9 gm NaCl in 100 gm of solution
Therefore, Given mass of solute (NaCl) = 0.9 gm
Molar mass of NaCl = 23 + 35.5 = 58.5 mol-1
Number of moles = Given mass (m) / Molar mass (M) = 0.9/ 58.5 = 0.015 u
Number of molecules
= number of moles( n) × Avogadro constant (N0)
= 0.015 × 6.022 × 1023
= 0.09033 × 1023 or 9.033 × 1021