Topic: Molecular weight?
June 16, 2019 / By Christin Question:
Hemoglobin is the oxygen-carrying molecule of red blood cells, consisting of a protein and a nonprotein substance. The nonprotein substance is called heme. A sample of heme weighing 35.2 mg contains 3.19 mg of iron. If a heme molecule contains one atom of iron, what is the molecular weight of heme?
Avalona | 10 days ago
Divide .00319 grams by the molecular weight of iron (55.8 grams/moles). This will give you the number of moles of material in the sample. (5.716x10e-5 moles). The sample weighs 35.2 mg (or 0.0352 grams). Molecular weight is grams/mole. So 0.0352/5.716x10e-5=615.72 grams/mole)
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Throughout my chemical life I have known that equivalent weight is not equal to molecular weight. What is the definition and examples of equivalent weight? Equivalent weight is the amount of an element that reacts, or is involved in reaction with, 1 mole of electrons. It is 'defined' by many texts as the weight of the element combining with 1 g hydrogen, 8 g oxygen or 35.5 g chlorine, each of which would either provide or accept one mole of electrons in a reaction. For example, hydrogen, with atomic weight 1.008 and valence 1, has an equivalent weight of 1.008. Oxygen assumes a valence of 2 and has an atomic weight of 15.9994, so it has an equivalent weight of 7.9997. What is the molecular weight? The molecular or molar mass of a substance is the mass of 6.022*10^23 molecules of the substance, in grams. (there is a very complicated difference between molar and molecular mass, but for practical purposes, we can consider these interchangeable) The distinction between these two values is as follows: If you prepare a solution of 1 litre containing 1 equivalent mass, the solution is called a Normal solution and is abbreviated, 1.00N. If you prepare a solution containing 1 molar mass of substance dissolved to a volume of 1 litre, then the solution then the solution is called a Molar solution, abbreviated, 1.00M. When dealing with monoprotic acids, such as HCl or HNO3 the molar mass is the same numerically as the equivalent mass. Therefore a 1N HCl is exactly the same as 1M HCl When dealing with a diprotric acid, such as H2SO4, where 1 molar mass has two reactable hydrogens, The 1N solution has half the concentartion of the 1M solution When you deal with a triprotic acid, such as H3PO4. the 1N solution will have 1/3 the concentration of a 1M solution. The same situation applies to bases: 1M NaOH is exactly the same as 1N NaOH. But 1M Mg(OH)2 is double the concentration of 1N Mg(OH)2. Your question: Why is it that the equivalent weight is equal to the molecular weight? It is not always equal.
Since heme:iron is a 1:1 ratio, look at the relative amounts of each:
35.2 / 3.19 = 11.034
Fe has atomic weight of 55.847
11.034 x 55.847 = 616.24
This constitutes a small portion of the whole structure: hemoglobin is usually a tetramer (4-molecule) with each subunit having a total MW of about 17,000