So far this week, we’ve learned that the coefficients of balanced equations provide us with mole ratios, which we can then use to predict the amount of products generated by reactants in a chemical reaction. Once we have a theoretical value for the amount of reactant we will generate (our theoretical yield), we can then run the experiment and measure how much reactant we actually recover (our actual yield). By comparing the theoretical and actual yeilds, we can calculate our percent yield: how much reactant we are able to recover given real-world experimental constraints.
In the Going Through the Mole Tunnel section, we investigated the reaction of copper with silver nitrate to produce silver and copper nitrate. We went on to calculate the number of moles of copper and silver nitrate we would have to combine to produce 30 g of silver. Now, imagine we actually did that experiment and our recovery of pure silver was 28.5 g. What was our percent yield of silver?
- Theoretical yield of silver = 30 g
- Actual yield of silver = 28.5 g
Percent yield = (actual yield) / (theoretical yield) x 100%
For our experiment: Percent yield = 28.5 / 30 x 100% = 95%
Not bad! Our actual yield was only 5% less than our theoretical yield. Why wasn’t the percent yield 100%? Experimental error! Maybe we mis-measured one of our reagents. Maybe we used a solution that wasn’t prepared as accurately as it should have been. Maybe our balance needs to be re-calibrated, or the volume measured by our beaker wasn’t perfectly accurate.