Problem #1: Calculate the molar mass of a metal that forms an oxide having the empirical formula M2O3 and contains 68.04% of the metal by mass. Identify the metal.
(1) 68.04% of the compound is the metal, therefore 31.96% is oxygen.
(2) The three oxygens weigh 48.0 and this represents 31.96% of the total weight.
(3) 31.96% is to 48.0 as 100% is to x
x = 150.2
(4) Subtract 48 from 150.2 to get 102.2, which is the weight contribution of M2.
(5) Dividing by two gets 51.1. The nearest atomic weight on the periodic table is 50.9, that of vanadium.
Problem #2: Hemoglobin is the oxygen carrying compound found in human blood. It is found to contain 0.3335% iron by mass. It is already known that one molecule of hemoglobin contains four atoms of iron. What is the molecular mass of hemoglobin?
(1) One iron weighs 55.845; four of them weigh 223.38
(2) This value (223.38) represents 0.3335% of the total mass of hemoglobin.
(3) 223.38 is to 0.3335% as x is to 100%
(4) x = 66,980 g/mol
Comment: in biochemistry, molecular weights into the tens of thousands are very common. Also, the unit "daltons," as a symbol for g/mol, is often used in biochemistry, as in 66,980 daltons.