Calculation of Molar Mass

What is a molar mass?

Molar mass is defined as, “The relative atomic mass/formula mass/molecular mass of an element/formula unit/compound expressed in grams”.

Simply put, molar mass is the mass (in grams) of one mole of any substance. Hence it shows how many grams does one mole of that particular substance have in it. Examples: 1) Molar mass of water Molar mass of water is 18.015 g/mol which reveals that one mole of water weighs roughly 18 grams. Since water has a molecular formula H2O, so its molar mass is calculated as below: Molar mass of water (H2O) = 2(mass of H in a.m.u) + 1(mass of O in a.m.u) = 2(1.008g/mol) + 1(15.999g/mol) = 18.015 g/mol 2) Molar mass of glucose Glucose has a molar mass of 180.156 g/mol which shows that one mole of glucose weighs around 180 grams. Since the molecular formula of glucose is C6H12O6, so its molar mass is calculated as follow: Molar mass of glucose (C6H12O6) = 6(mass of C in a.m.u) + 12(mass of H in a.m.u) + 6(mass of O in a.m.u) = 6(12.011g/mol) + 12(1.008g/mol) + 6(15.999 g/mol) = 180.156 g/mol 3) Molar mass of sodium carbonate (Na2CO3) Molar mass of sodium carbonate is 105.988 g/mol which reveals that one mole of sodium carbonate weighs roughly 106 grams. Since sodium carbonate has a molecular formula Na2CO3, so its molar mass can be calculated as: Molar mass of Na2CO3 = 2(mass of Na in a.m.u) + 1(mass of C in a.m.u) + 3(mass of O in a.m.u) = 2(22.990g/mol) + 1(12.011g/mol) + 3(15.999g/mol) = 105.988 g/mol Steps for calculating the molar mass Molar mass of any substance can be calculated in 3 easy steps: Step-1: Figure out the chemical formula (not the empirical formula) of the substance whose molar mass is to be calculated. Then from this chemical formula identify the individual atoms/ions/formula units which makeup that substance For example the chemical formula of glucose is C6H12O6. And there are three individual atoms that make up this formula, named Carbon (C), Hydrogen (H) and Oxygen (O). Step-2: Look for the relative atomic/formula mass of every constituent atom/formula unit in the periodic table. Then multiply this mass with the number of units of that particular atom/formula unit. Also replace a.m.u (relative atomic mass) with g/mol (molar mass) For example looking for the values of relative atomic masses of carbon (C), hydrogen (H) and oxygen (O) in the following periodic table Relative atomic mass of C = 12.011 a.m.u = 12.011 g/mol Relative atomic mass of H = 1.008 a.m.u = 1.008 g/mol Relative atomic mass of O = 15.999 a.m.u = 15.999 g/mol So, 6 C=6×12.011=72.066 g/mol 12 H=12×1.008=12.096 g/mol 6 O=6×15.999=95.994 g/mol Step-3: Add up all the values and then final value is the molar mass, with the units “g/mol” For example, Molar mass of C6H12O6 = 6(Mass of C in a.m.u) + 12(Mass of H in a.m.u) + 6(Mass of O in a.m.u) = 6(12g/mol) + 12(1.01g/mol) + 6(16g/mol) = 72.066 g/mol + 12.096 g/mol + 95.994 g/mol Molar mass of C6H12O6 = 180.156 g/mol