94. Solving a chemistry problem

The content of the main active ingredient ranitidine in the drug Rantak solution for injection 25 mg / ml is determined by HPLC, measuring the peak areas S and S (st) on the chromatograms of the analyzed and standard solutions.
Rantak contains ranitidine hydrochloride C13H22N4O3S*HCl; molecular weight of ranitidine M1=314.40, molecular weight of ranitidine hydrochloride M2=350.86.
To prepare the analyzed solution of the drug, add 1 ml of the Rantak preparation to a 25 ml volumetric flask, bring the volume of the solution in the flask to the mark with a mixture of methanol and 0.1 mol/l of an aqueous solution of ammonium acetate (in a volume ratio of 85:15) and mix. Take 1 ml of the resulting solution, dilute with the same mixture of methanol and ammonium acetate solution to 10 ml and mix. Get the analyzed solution.
To obtain a standard solution, m = 56 mg of a standard sample of ranitidine hydrochloride (equivalent to 50 mg of ranitidine) is added to a 50 ml volumetric flask, the solution in the flask is brought to the mark with a mixture of methanol and 0.1 mol/l of an aqueous solution of ammonium acetate (85:15 ) and mix. Take 1 ml of the resulting solution and dilute to 10 ml with the same mixture of methanol and ammonium acetate. Get a standard solution.
Equal amounts (10 μl) of the analyzed and standard solutions are chromatographed under the same conditions and the peak areas S and S(st) are measured on the chromatograms of the analyzed and standard solutions, respectively.
Derive the following formula to calculate the W content of ranitidine in Rantak in mg/ml:
X=
Where A is the percentage of ranitidine hydrochloride in the standard sample.
Calculate the content of ranitidine in Rantak in mg/ml if A=99.99%; S:S(st)=1.03.
Answer: 25.84 mg / ml.