How many carbon atoms are contained in 84.3 g of ethyne (c2h2) ?

To arrive at an answer in units of carbon atoms, first we must cancel out the grams by diving 84.3g C2H2 by the molar mass of C2H2 (26.038g/mol). This molar mass is calculated by adding the masses of 2 carbon atoms (12.011g/mol) and 2 hydrogen atoms (1.008g/mol). This renders units of moles of ethyne. Next multiply by Avagadro's number (6.022x10^23 atoms/mol) to cancel the moles of ethyne and reach atoms of ethyne. Finally, because there are 2 carbon atoms in C2H2, multiply by 2 atoms Carbon/1 atom C2H2 to reach the final answer of 3.90x10^24 atoms of Carbon. This answer has been rounded to three significant figures because the lowest known value of significant figures throughout the calculations was three (84.3). The entire process is as follows: 84.3g C2H2 x (1 mol C2H2) / (26.038 g C2H2) x (6.022x10^23 atoms C2H2) / (1 mol C2H2) x (1 atoms C2H2) / (2 atom C) = 3.90x10^24 atoms C