Per the amu definition, a single 12C atom weighs 12 amu (its atomic mass is 12 amu). (credit: modification of work by Mark Ott).īecause the definitions of both the mole and the atomic mass unit are based on the same reference substance, 12C, the molar mass of any substance is numerically equivalent to its atomic or formula weight in amu.

It is convenient to consider 1 mol of C 9 H 8 O 4 and use its molar mass (180.159 g/mole, determined from the chemical formula) to calculate the percentages of each of its elements: C 9 mol C × molar mass C molar mass C 9 H 8 O 4 ×. Finding molar mass starts with units of grams per mole (g/mol). To calculate the percent composition, the masses of C, H, and O in a known mass of C 9 H 8 O 4 are needed. This is the number of moles in 100 g of water. Divide the mass of your sample by the molar mass: 100 g / 18.015 g/mol 5.551 mol. To calculate the percent composition, the masses of C, H, and O in a known mass of C 9 H 8 O 4 are needed. From left to right (bottom row): 32.1 g sulfur, 28.1 g silicon, 207 g lead, and 118.7 g tin. Molecular weight calculation: 12.0107 + 32.0652. Calculate the molar mass of water, which is two hydrogen atoms and one oxygen atoms molar masses combined: (2 × 1.008 g/mol) + 15.999 g/mol 18.015 g/mol. From left to right (top row): 65.4 g zinc, 12.0 g carbon, 24.3 g magnesium, and 63.5 g copper.

Carbon molar mass grams