YAY LET'S TALK ABOUT PERCENTS! (And no not your grade percentages)

It's all about the ratios and percentages today! We'll be looking at percent YIELD and percent PURITY.


You're probably wondering, "WHAT IS THE DIFFERENCE? I DON'T EVEN-"
It's okay. Calm your horses. We will explain, don't worry.


PERCENT YIELD

• measures amount of product actually produced compared to the amount that would theoretically be produced by using stoichiometry
• often, there will be an excess of reactants (because in the real world, we seldom have ideal reactions) so the products resulted will always be less than what is expected; so percent yield is needed
• determines what percentage of yield is expected in a reaction 
• ratio of actual yield (amount of product obtained in grams) and theoretical yield (amount of product expected in grams)

NOTE: Remember to always round to one decimal place when dealing with percentages!

Example: Fe₂O_3(s) + 3CO_(g) → 2Fe_(l) + 3CO_2(g)

If 2.2g of Iron (Fe) is produced when 6.5g of Fe₂O₃ is reacted with excess CO, what is the percent yield?

Step 1: Find the expected (theoretical) amount of Fe to be produced by 15.0g Fe₂O_3.

(Using stoichiometry - remember to use the ratios of the elements/compounds!)

6.5g Fe₂O₃ x 1mol Fe₂O₃ /159.6g Fe₂O₃ x 2mol Fe / 1mol Fe₂O₃ x 55.8g Fe/ 1mol Fe

= 4.545g Fe is expected to be produced

Step 2: Calculate the % yield using the ratio formula.

We know that 2.2g of Fe is actually produced, and 4.545g Fe is expected

Therefore....

2.2g Fe/4.545g Fe x 100% = 48.4% of Fe is actually produced for every 100% predicted

If, for example, the percent yield is given and the amount of a reactant that reacts is also given, how would you find the number of grams a certain product will be produced?

Let's use the same equation: Fe₂O_3(s) + 3CO_(g) → 2Fe_(l) + 3CO_2(g)

The percent yield is 70.0% and 3.10g of CO is reacted with excess Fe₂O₃. Find the number of grams CO₂ produced.

Step 1: Calculate amount of the product expected.

3.10g CO x 1mol CO/ 28.0g CO x 3mol CO₂/3mol CO x 44.0g CO₂/1mol CO₂

_{= 4.871g CO2 is expected}

Step 2: Turn the percentage into a decimal and calculate the amount of product produced.

0.700 = amount of product produced / 4.871g

amount of product produced = 0.700 x 4.871g

= 3.41g CO<sub>2 produced 


ET VOILA! Ergo, you have your grams of the actual product produced! Easy, right?</sub>

_{OMG, if this doesn't make you laugh... you need a sense of humour. (Can I get an "awww")}

_{PERCENT YIELD}

• used to calculate the amount of purity in a reactant (since not all reactants are pure)
• this is needed to find the amount of reactant that is actually available (pure) in order to react and form products

Percent Purity=   Mass of pure substance    x 100

                     Mass of impure sample

Example: 1Cu + 1Cl2 > 1CuCl2

A 7.4g copper ore contains 3.2g of pure copper. What's the percent purity?

% purity = 3.2g / 7.4g x 100%

= 43.2% pure copper

By finding the grams of pure copper (which is already stated here), you can find how much of a certain product is produced. That's why % purity is important! :)