Short answers, pulled from the story.
Mozart's grave at St. Marx Cemetery is unmarked because Emperor Joseph II decreed that all burials follow this practice, not because Mozart's family was poor. The regulation applied to every interment at the cemetery when Mozart was buried there in 1791.
St. Marx Cemetery is located in the Landstraße district of Vienna, Austria. It was established outside the city's outer walls following a decree by Emperor Joseph II in 1784.
St. Marx Cemetery opened in 1784, following a decree by Emperor Joseph II, and remained in use until 1874. It was later restored and opened to the public in 1937.
The memorial to Mozart that stands at St. Marx Cemetery today was refurbished by Viennese sculptor Florian Josephu-Drouot in 1950. It evolved from a gravestone erected in 1855 and a later memorial tablet installed by a cemetery worker.
The gravestone placed in 1855 at the presumed site of Mozart's grave was later transferred to the Zentralfriedhof in Vienna, where it was placed among the graves of famous musicians. A memorial tablet was installed at St. Marx in its place.
St. Marx Cemetery contains the graves of Anton Diabelli, Johann Georg Albrechtsberger, Anna Gottlieb, Josef Strauss, Elias Parish Alvars, Alexander Ypsilantis, Count Philipp von Cobenzl, and Franz Xaver Sussmayr, among others. Sussmayr's grave, like Mozart's, is unmarked.