Apr. 4 Percent Yield & Percent Purity

 Percent Yield

What is percent yield? It is the ratio.  A ratio of what? It is a ratio of amount of product OBTAINED to the amount of product EXPECTED by calculation.  It must be expressed at a %. 

Here is an example:
By using this reaction: C3H6Cl2 + 2 KSCN -> C3H6(SCN)2 + 2KCL

The percent yield is 52.0%.  How many grams of C3H6(SCN)2 will be produced if 1.5 g of C3H6Cl2 is reacted with excess KSCN?

First, we got to calculate the amount of C3H6(SCN)2 expected to be produced when 120 g of C3H6Cl2 reacts.

1.5 g C3H6Cl2 x 1 mol C3H6Cl2   x 1 mol C3H6(SCN)2  x  158 g C3H6(SCN)2
                            113 g C3H6Cl2     1 mol C3H6Cl2             1 mol C3H6(SCN)2

= 2.10 g of C3H6(SCN)2

Now that we have the amount of C3H6(SCN)2 expected, we have the bottom of the fraction, the predicted mass of the product.  We know that the end percent yield is 52%, we will have to solve for the top.

% yield = C3H6(SCN)2 obtained  x 100% = 52.0 % yeild
                2.10 g of C3H6(SCN)2

52.0 x 2.10 = 109.2/100 = 1.09 g of C3H6(SCN)2 obtained.

Percent Purity
It is the ratio of the mass of impure substance to the mass of impure substance.  The fraction is just like percent yield but on top is mass of pure substance over mass of impure substance all multiplied by 100.

If a 5.00 g sample of iron ore contains 0.25 g of iron metal, what is the percent purity?

Mass of pure substance     x 100 = percent purity
Mass of impure substance

0.25 g Iron         x 100% = 5.00%
5.00 g Iron ore

Percent Yield:

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