The molar mass of hemoglobin is about65000 g ${\mathrm{mol}}^{-1}$ . Every hemoglobin contains 4 iron atoms. Thus,

1 mole of hemoglobin contains 224 g iron

1 mole of hemoglobin contains 56 g iron

iron content in hemoglobin is 0.35% by mass

if iron content is increased to 0.56%, molar mass of hemoglobin would be higher than 65000 g ${\mathrm{mol}}^{-1}$