Percent Yield??

Question

The percent yield of the reaction of aluminum with hydrochloric acid is 72%, how much aluminum should be reacted in order to recover

90 grams of aluminum chloride.

I figured out how to calculate percent yield, but I am having trouble figuring out how to go backwards when given percentage?

Answer 1

A percent yield of 72% means that for a given amount of Al, you get out only 72% of the aluminum chloride that you would've expected. This means that the initial amount of Al must increase to compensate for the 28% not recovered.

I've set up the dimensional analysis below with x representing the amount of Al in grams needed. You'll have to replace the question marks with molar masses and molar ratios from the balanced chemical reaction:

x g Al * (1 mol Al/ ? g Al) * (? mol AlCl₃ / ? mol Al) * (72%/100%) = 90 g

x g Al = 90 g/[(1 mol Al/ ? g Al) * (? mol AlCl₃ / ? mol Al) * (72%/100)]

Hope that helps! :)