# Question #f79bb

Oct 26, 2014

In order to calculate the mass of 2.1 moles of acetylene, you must first determine the mass of one mole, which is its molar mass. The mass of one mole of a substance is its atomic weight in grams/mole (g/mol). Then multiply the molar mass times the number of moles given in the question.

To determine the molar mass of ${\text{C"_2"H}}_{2}$, multiply the atomic weights as g/mol for carbon and oxygen times the number of atoms of each element in the compound, then add them together.

Molar mass of ${\text{C"_2"H}}_{2}$.

C: 12.011g/mol x 2 = 24.022g/mol

H: 1.008g/mol x 2 = 2.016g/mol

Total: 26.038g/mol

Molar mass of ${\text{C"_2"H}}_{2}$ is 26.038g/mol

Calculate the mass of 2.1 mol ${\text{C"_2"H}}_{2}$.

${\text{2.1 mol C"_2"H}}_{2}$ x $\text{26.038g"/"1 mol}$ = $\text{54.6798g/mol}$

The mass of 2.1 moles ${\text{C"_2"H}}_{2}$ is 54.6798g, which is rounded to 55g to match the least number of significant figures, which is two in 2.1 moles.