**If I have 10 g of a solution made up of PCl**_{3}and PCl_{5}and 81.04% of the mixture by mass is due to Cl, how many grams of PCl_{3}and PCl_{5}is there? I don't think there is enough information here!

Ed Tremblay 11/22/99-
There are two unknowns (grams of PCl
_{3}and grams of PCl_{5}) so you'll have to write two independent equations to solve the problem. If there's nothing in the mixture but PCl_{3}and PCl_{5},10 g = grams of PCl

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 PCl_{3}+ grams of PCl_{5}

_{3}plus the mass of Cl in the PCl_{5}. 77.45% of PCl_{3}and 85.13% of PCl_{5}is chlorine, so:8.104 g Cl = 0.7745 × g of PCl

Now all you have to do is solve the two equations simultaneously (e. g. solve the first equation for PCl_{3}+ 0.8513 × g of PCl_{5}_{3}and substitute the result into the second equation). Can you take it from here?

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

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