In this video we will cover how to do stoichiometry, and how to calculate the
percent yield of the products. Created by the Writing and Tutoring Center at
Tacoma Community College. I have a chemical reaction: sodium reacts with
water and produces sodium hydroxide and hydrogen gas. And the question is if you
have three grams sodium, how many grams sodium hydroxide will you make. And to
make the following steps easier, I'm going to summarize a question as
below. And the first step when we are trying to do stoichiometry problems is
always balance the reaction. This reaction that they have given us is not
balanced yet, so we need to balance it by going through the process that I have
mentioned in a previous video. And the link to that video will be in the
description box. Now let's stop the video for a few minutes and try to balance
this reaction yourself, and hit Play when you're ready. And this is the balanced
reaction that I've got. Now we can start doing stoichiometry and find how many grams
of sodium hydroxide that we get from 3 grams of sodium. And step two when we do
the stoichiometry problems is to do unit conversion, converting from the given
units to the desired units, and in this case is converting from grams of sodium
to grams of sodium hydroxide. And the method that we can use to go through
this step was also mentioned in my unit conversion video and the link to that
video will also be in the description box. Now let's stop the video for a few
minutes and try to convert from grams of sodium to grams of sodium hydroxide
yourself, and hit Play when you're ready.
So the first thing that I always do is to write an equation which is an equal
sign. And on the left side of the equation I'm gonna put out my given
units and on the right side of the equation, I'm gonna put out my desired
unit, and establish a train track. After that, I observe that I have sodium, but I
want sodium hydroxide as the desired unit so I need to find the relationship
between these two compounds. We can use the reaction that we have balanced
before to find the mole ratio between these two compounds. Based on the
coefficient in front of the compounds, we have two moles of sodium equivalence to
two moles of sodium hydroxide. And next step is I'm going to find the
relationship between grams of sodium and moles of sodium to establish my next
conversion factor. And whenever we want to find the relationship between grams
and moles of the same compound, we will use the molecular weight of that
compound, so I'm going to use a molecular weight of sodium which is 22.99
grams per mole to establish my next conversion factor. Next step will be
finding the relationship between moles of sodium hydroxide and grams of sodium
hydroxide. As mentioned before, I'm going to use a molecular weight of sodium
hydroxide which is 39.999 grams per mole. Now the last step is to check the
cancelling unit. Grams of sodium will be cancel out, moles of sodium is also
cancel out, moles of sodium hydroxide is also cancel out. So on the left side of
the equation we are left with grams of sodium hydroxide, and that is also our
desired units. Now we need to put all of the numbers in the calculator and find
the answer for this unit conversion step.
5.219530 is what I got. And don't forget to consider significant figures.
We have 3.00 grams of sodium which has three significant figures, so our answer
also needs to have three significant figures. And that is called theoretical
yield. So we have three grams of sodium, and through our calculation we expect to
get 5.22 grams of sodium hydroxide after the reaction is done.
However, in lab you only get 5.05 grams of sodium hydroxide, then
what is the percent yield of this compound? I will give you the general
formula of finding the percent yield which is equal to the actual yield
divided by the theoretical yield and time a hundred percent. Now let's stop the
video for a few minutes and try to find the percent yield from the given
information and hit Play when you're ready. From our calculation, we found that
our theoretical yield is 5.22 grams of sodium hydroxide. And
from the question they're giving us, the actual yield that we got in lab was 5.05
grams of sodium hydroxide. Therefore, when I establish my
percent yield equation, I will put 5.05 grams on top and 5.22 grams
on the bottom. And this is a percent yield that I got. Don't forget
to consider significant figures when you calculate the percent yield. If you have
any further questions, you can go to the Writing and Tutoring Center for more
help. And if you need more practice problems, please refer to the Writing and
Turing Center handouts, or the chemistry 110 resource website.
Thank you for watching and good luck!
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