111. Solving a chemistry problem

50. A sample of the alloy weighing 0.1938 was dissolved in hydrochloric acid and magnesium was precipitated with sodium hydrogen phosphate in an ammonium buffer medium. The precipitate was dissolved in 50.00 ml of 0.1 M HCl (K=0.9981); 18.00 ml of NaOH solution (T(NaOH)=0.004000) was used for back titration with methyl orange. Determine the mass fraction (%) of Mg in the alloy. Answer: 20.01%.