f[x_] := x^2 + Sin[x]
f'[x]

deltaSmix[xA_] = -n*
          r*(xA*Log[xA] + (1 - xA)*
                Log[1 - xA]);
soln1 = deltaSmix'[xA] // Simplify

soln2 = Solve[soln1 == 0, xA]
xAmax = soln2[[1, 1, 2]]

molarRatio = 1;
molarMass1 = 12*6 + 6;  
molarMass2 = 
      12*6 + 5 + 29;   
molWtRatio = 
      molarMass1/molarMass2;
massRatio = molarRatio*molWtRatio // N