What was the capital of the union of provinces in the Río de la Plata comprised by most of the former dependencies from the last to be organized and also the shortest-lived of the Viceroyalties of the Spanish Empire in the Americas?
United Provinces of the Rio de la Plata:  It comprised most of the former Viceroyalty of the Río de la Plata dependencies and had Buenos Aires as its capital.
i don't know