The molar mass of a substance is the mass of one moles of the substance
For exampel;Ethanol whose molar mass is C2H6O has a molecular mass of 46.1 amu
Now we know how to find the mass of one mole of substance,there two important questions to ask.First,how much does a given number of moles of a substance weight?Second,how many moles of a given formula unit does a given mass of substance contains?Both questions are easily answered using dimensional analysis or the conversion -factor method.
To illustrate,consider the conversion of moles of ethanol to mass in grams of ethanol
1 mol C2H5OH = 46.1 g C2H5OH
Thus,the factor converting moles of ethanol to grams of ethanol is 46.1 g C2H5OH/ 1 mol C2H5OH.Note that the unit you are converting from is one the bottim of the conversion factor; the unit you are converting to is on the top.
For example: Determine the grams of zinc iodide from the 0.0654 mol of ZnI2.
Solution: Use the formula weight of znI2 to write the factor that converts from mol of ZnI2 to g ZnI2.Note that the unit you are converting from is on the bottom of the conversion factor.
Molar mass of ZnI2 is 319 amu, (which is obtained by summing the atomic weight in the formula).Therefore,
1 mol ZnI2 = 319 amu
conversion factors are : 1 mol ZnI2/319 g ZnI2 or 319 g ZnI2/ 1 mol ZnI2
given x conversion factor
0.0654 mol ZnI2 x 319 g ZnI2/ 1 mol ZnI2 = 20.9 g ZnI2