# How do I compute percent compound in a mixture, given mixture mass and element percents?

If I have 10 g of a solution made up of PCl3 and PCl5 and 81.04% of the mixture by mass is due to Cl, how many grams of PCl3 and PCl5 is there? I don't think there is enough information here!
Ed Tremblay 11/22/99

There are two unknowns (grams of PCl3 and grams of PCl5) so you'll have to write two independent equations to solve the problem. If there's nothing in the mixture but PCl3 and PCl5,

10 g = grams of PCl3 + grams of PCl5

You know that 81.04% of the mass of the mixture is Cl, so there are a total of 8.104 g of Cl in the sample. The total mass of chlorine has to be equal to the mass of Cl in the PCl3 plus the mass of Cl in the PCl5. 77.45% of PCl3 and 85.13% of PCl5 is chlorine, so:

8.104 g Cl = 0.7745 × g of PCl3 + 0.8513 × g of PCl5

Now all you have to do is solve the two equations simultaneously (e. g. solve the first equation for PCl3 and substitute the result into the second equation). Can you take it from here?

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! How do I compute percent compound in a mixture, given mixture mass and element percents?