Question: A certain sample of phosphate rock contains 26.26% P2O5. A .5428 gram sample is analyzed by percipitating MgNH4PO4*6H2O and heating it strongly to form Mg2P2O7. compute the mass of Mg2P2O7.

Question

I'm not entirly sure what to do. Do you set up an equation?

Answer 1

You may want to write equations but it isn't necessary.
I would convert 0.5428 g sample to g P2O5.
0.5428 x 0.2626 = ??
Then ?? x (1 mol P2O5/molar mass P2O5) converts that to say x mols P2O5.
If we then recognize that 1 mole P2O5 must be present to make 1 mole Mg2P2O7, then x mol P2O5 x (1 mol Mg2P2O7/1 mole P2O5) x (molar mass Mg2P2O7/1 mol Mg2P2O7) should give grams Mg2P2O7. Check my work. There is an easier way to do this but it isn't taught anymore. The OLD way (when I was a student) was to use a gravimetric factor. That converts directly as
0.5428 x 0.2626 x (molar mass Mg2P2O7/molar mass P2O5) = ??
I get something like 0.22 something.