In May 1915, Lassen Peak, California, the southernmost active volcano in the Cascade Range, erupted explosively. Avalanches, mudflows, and flows of hot ash and gas devastated nearby areas, and volcanic ash fell as far away as 200 miles to the east. The Lassen area remains volcanically active, and the volcano hazards demonstrated in 1915 still can threaten not only nearby areas but also more distant communities. Recent work by scientists with the U.S. Geological Survey (USGS) in cooperation with the National Park Service is shedding new light on these hazards.
The percent composition by mass of an element is determined by dividing the molar mass of the element by the molar mass of the compound. In this case, the molar mass of magnesium is 24. Hence the answer to this problem is A. 24/184 x 100
