Question

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) 

Answer 1

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