Question 4.11: Mass Percent Composition Calculate the mass percent of Cl in...
Mass Percent Composition
Calculate the mass percent of Cl in Freon-112 (C_2Cl_4F_2), a CFC refrigerant.
SORT You are given the molecular formula of freon-112 and asked to find the mass percent of Cl.
GIVEN: C_2Cl_4F_2
FIND: mass percent Cl
STRATEGIZE The molecular formula tells you that there are 4 mol of Cl in each mole of Freon-112. Find the mass percent composition from the chemical formula by using the equation that defines mass percent. The conceptual plan shows how to use the mass of Cl in 1 mol of C_2Cl_4F_2 and the molar mass of C_2Cl_4F_2 to find the mass percent of Cl.
CONCEPTUAL PLAN
mass % Cl=\frac{4\times molar \ mass \ Cl}{molar \ mass \ C_2Cl_4F_2}\times 100 \%
RELATIONSHIPS USED
mass percent of element X==\frac{mass \ of \ element \ X \ in \ 1 \ mol \ of \ compound}{mass \ of \ 1 \ mol \ of \ compound}\times 100 \%
SOLVE Calculate the necessary parts of the equation and substitute the values into the equation to find mass percent Cl.
4 × molar mass Cl = 4(35.45 g/mol) = 141.8 g/mol
molar mass C_2Cl_4F_2 = 2(12.01 g/mol) + 4(35.45 g/mol) + 2(19.00 g/mol)
= 24.02 g/mol + 141.8 g/mol + 38.00 g/mol = 203.8 g/mol
mass % Cl==\frac{4 \times molar \ mass \ Cl}{molar \ mass \ C_2Cl_4F_2}\times 100\%=\frac{141.8 \ \cancel{g/mol}}{203.8 \ \cancel{g/mol}} \times 100\%=69.58 \%
CHECK The units of the answer (%) are correct. The magnitude is reasonable because (1) it is between 0 and 100% and (2) chlorine is the heaviest atom in the molecule and there are four of them.