If know now the molar ratios you can calaculate the x in the equation.

But with this kind of reactions you cant be 100 % sure that all Fe is buidl into your Fe-dope-AluOx. To be really sure you would have to do an element analysis.

Calculation the "x" in your doping process from the initial amount of chemicals might greatly differ from the actual doping level of hematite with Al, considering that probably not all of the Al has integrated into the Fe2O3 crystal structure, and some of it might have been washed away in the final steps of the synthesis process. I assume you are using solvothermal process (I'm unsure because I can't read well the writing). As Thomas Mayer-Gall suggested, a good idea is to check the composition with EDS spectrometer under SEM or TEM microscope, which will give you true experimental values of the doping level. However, that will only work if you are certain of the mono-phase character of your samples. In case you have other oxide phases present, like Al2O3, which may form as well, EDS will not tell you the true level of Fe2O3 doping.

I suggest to check the samples with Mössbauer spectrometer, since the doping of Fe2O3 with Al has been thoroughly analyzed with this particular method.

Article Aluminous hematite: A m�ssbauer study

this is just one example.

Thank you for all your help. I calculated the value of X from the Rietveld analysis of X-ray diffraction and matched it with the value obtained from the elemental analysis using EDAX in TEM.