Percent Yield

AnnaWhen a chemistry experiment gives you some product, how do you tell whether that amount is a lot or a little? Percent yield gives us a way to compare what we collected with the maximum amount the reaction could produce.

MarcoRight. But before we calculate it, we need to separate two ideas. Actual yield is the amount of product collected. It’s usually measured as a mass, after the reaction and any separation or purification. Theoretical yield is different. It’s calculated from the balanced equation and starting quantities.

AnnaSo theoretical yield isn’t a promise about what you’ll collect. It’s the maximum possible amount if the limiting reagent reacts completely and none of the product is lost. How do we find that limiting reagent?

MarcoConvert each reactant quantity to moles. Then use the balanced equation to work out how much product each reactant could form. The reactant that could form the smaller amount of product runs out first. That’s the limiting reagent, and it determines the theoretical yield.

AnnaLet’s use the nitrogen and hydrogen example. The balanced equation is N two plus three H two yields two N H three. We start with twenty-eight point zero grams of nitrogen and three point zero zero grams of hydrogen. What do those amounts become in moles?

MarcoUsing the given molar masses, that’s one point zero zero mole of nitrogen and one point four eight eight moles of hydrogen. The equation needs three moles of hydrogen for each mole of nitrogen. So one mole of nitrogen would need three moles of hydrogen, but we have only one point four eight eight. Hydrogen is limiting, and nitrogen is in excess.

AnnaOnce we know that, we use hydrogen to calculate the maximum ammonia. Three moles of hydrogen produce two moles of ammonia. So one point four eight eight times two, divided by three, gives zero point nine nine two mole of ammonia. Then we convert that to grams, using seventeen point zero three grams per mole. That gives a theoretical yield of sixteen point nine grams.

MarcoAnd if the experiment actually collects twelve point seven grams, how do we compare the two?

AnnaDivide actual yield by theoretical yield, then multiply by one hundred. So twelve point seven divided by sixteen point nine, times one hundred, is seventy-five point one percent. The collected mass is seventy-five point one percent of the calculated maximum.

MarcoThe units matter, too. Actual and theoretical yield have to refer to the same product and use the same units. Here, both are grams of ammonia. And we divide actual by theoretical, not the other way around.

AnnaExactly. It’s also a mistake to use the excess reactant for the theoretical yield, or to ignore the balanced equation’s mole ratios. And theoretical yield doesn’t mean that amount must be collected in a real experiment.

MarcoPercent yield is commonly below one hundred percent because a reaction may not go to completion, or some product may be lost. So it tells us how the collected amount compares with the calculated maximum, while keeping the limiting reagent and the calculation straight.