how many grams of phosphorus pentachloride are needed to produce 49 grams of phosphoric acid.
get the balanced equation reaction of PCl5 and phosphoric acid.
from the balanced equation, obtain the mole ratio between the two compounds.
convert the mass of phosphoric acid to moles using n=mass/molar mass
determine the mole of PCl5 using the mole ratio
covert the mole of PCl5 to the mass using m=nMr.
hope that helps