Importance of basis: A liquid salt solution contains 5%(by mass)NaCl,7%(by mass)
ID: 701430 • Letter: I
Question
Importance of basis: A liquid salt solution contains 5%(by mass)NaCl,7%(by mass)MgCl2)3% by mass)KCl and 85%(by mass)water(H2O). What are the average molecular weight of the solution(salt plus water). What is the average molecular weight of the water free of solution( that is , the average molecular weight of the NaCL,MgCL2, and KCl components of the solution)? Finally, if the specific gravity of the solution (salt and water) is 1.14, what are the concentrations of each of the salts(NaCl,MgCl2,KCl) in g/L
Explanation / Answer
Part a
Basis - 100 g of salt solution
Moles of NaCl = mass/molecular weight
= 5/58.4 = 0.0856 moles
Moles of MgCl2 = 7/95.21 = 0.0735
Moles of KCl = 3/74.55 = 0.0402
Moles of H2O = 85/18 = 4.722
Total Moles = 0.0856 + 0.0735 + 0.0402 + 4.722
= 4.9213
Mol fraction of NaCl X1 = moles of NaCl/total moles
= 0.0856/4.9213 = 0.01739
Mol fraction of MgCl2 X2 = moles of MgCl2/total moles
= 0.0735/4.9213 = 0.01493
Mol fraction of KCl X3= moles of KCl/total moles
= 0.0402/4.9213 = 0.00816
Mol fraction of H2O X4= moles of H2O/total moles
= 4.722/4.9213 = 0.9595
Average molecular weight of salt
= M1*X1 + M2*X2 + M3*X3 + M4*X4
= 58.4*0.01739 + 95.21*0.01493 +74.55*0.00816 + 18*0.9595
= 20.3163
Part b
Total Moles = 0.0856 + 0.0735 + 0.0402
= 0.1993
Mol fraction of NaCl X1 = moles of NaCl/total moles
= 0.0856/0.1993 = 0.4295
Mol fraction of MgCl2 X2 = moles of MgCl2/total moles
= 0.0735/0.1993 = 0.3687
Mol fraction of KCl X3= moles of KCl/total moles
= 0.0402/0.1993 = 0.2017
average molecular weight of the water free of solution
= M1*X1 + M2*X2 + M3*X3
= 58.4*0.4295 + 95.21*0.3687 +74.55*0.2017
= 75.223
Part C
Density of solution = 1.14*1000 = 1140 g/L
Concentration of NaCl = 0.01739 * 1140 g/L = 19.8246 g/L
Concentration of MgCl2 = 0.01493 * 1140 g/L = 17.0202 g/L
Concentration of KCl = 0.00816 * 1140 g/L = 9.3024 g/L
Concentration of H2O = 0.9595 * 1140 g/L = 1093.83 g/L