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- Gravimetric analysis and precipitation gravimetry
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Definition of precipitation gravimetry, and an example of using precipitation gravimetry to determine the purity of a mixture containing two salts.
What is precipitation gravimetry?
Precipitation gravimetry is an analytical technique that uses a precipitation reaction to separate ions from a solution. The chemical that is added to cause the precipitation is called the precipitant or precipitating agent. The solid precipitate can be separated from the liquid components using filtration, and the mass of the solid can be used along with the balanced chemical equation to calculate the amount or concentration of ionic compounds in solution. Sometimes you might hear people referring to precipitation gravimetry simply as gravimetric analysis, which is a broader class of analytical techniques that includes precipitation gravimetry and volatilization gravimetry. If you want to read more about gravimetric analysis in general, see this article on gravimetric analysis and volatilization gravimetry.
In this article, we will go through an example of finding the amount of an aqueous ionic compound using precipitation gravimetry. We will also discuss some common sources of error in our experiment, because sometimes in lab things don't go quite as expected and it can help to be extra prepared!
Example: Determining the purity of a mixture containing
Oh no! Our sometimes less-than-helpful lab assistant Igor mixed up the bottles of chemicals again. (In his defense, many white crystalline solids look interchangeable, but that is why reading labels is important!)
As a result of the mishap, we have
of a mysterious mixture containing and . We would like to know the relative amount of each compound in our mixture, which is fully dissolved in water. We add an excess of our precipitating agent silver(I) nitrate, , and observe the formation of a precipitate, . Once the precipitate is filtered and dried, we find that the mass of the solid is .
What is the mass percent of
in the original mixture?
Any gravimetric analysis calculation is really just a stoichiometry problem plus some extra steps. Since this is a stoichiometry problem, we will want to start with a balanced chemical equation. Here we are interested in the precipitation reaction between
and to make , when is in excess.
You might remember that precipitation reactions are a type of double replacement reaction, which means we can predict the products by swapping the anions (or cations) of the reactants. We might check our solubility rules if necessary, and then balance the reaction. In this problem we are already given the identity of the precipitate,
. That means we just have to identify the other product, , and make sure the overall reaction is balanced. The resulting balanced chemical equation is:
The balanced equation tells us that for every
, which is the compound we are interested in quantifying, we expect to make , our precipitate. We will use this molar ratio to convert moles of to moles of . We are also going to make the following assumptions:
- All of the precipitate is
. We don't have to worry about any precipitate forming from the .
- All of the
has reacted to form . In terms of the stoichiometry, we need to make sure we add an excess of the precipitating agent so all of the from reacts.
Now let's go through the full calculation step-by-step!
: Convert mass of precipitate, to moles
Since we are assuming that the mass of the precipitate is all
, we can use the molecular weight of to convert the mass of precipitate to moles.
: Convert moles of precipitate to moles of
We can convert the moles of
, the precipitate, to moles of using the molar ratio from the balanced equation.
: Convert moles of to mass in grams
Since we are interested in calculating the mass percent of
in the original mixture, we will need to convert moles of into grams using the molecular weight.
: Calculate mass percent of in the original mixture
The mass percent of
in the original mixture can be calculated using the ratio of the mass of from Krok and the mass of the mixture.
Shortcut: We could also combine Kroks
through into a single calculation which will involve careful checking of units to make sure everything cancels out properly:
Potencjalne źródła błędu
We now know how to use stoichiometry to analyze the results of a precipitation gravimetry experiment. If you are doing gravimetric analysis in lab, however, you might find that there are various factors than can affect the accuracy of your experimental results (and therefore also your calculations). Some common complications include:
- Lab errors, such as not fully drying the precipitate
- Stoichiometry errors, such as not balancing the equation for the precipitation reaction or not adding
What would happen to our results in the above situations?
: The precipitate is not fully dried
Maybe you ran out of time during the lab period, or the vacuum filtration set-up was not producing sufficient vacuum. It probably doesn't help that water is notoriously difficult to fully remove compared to typical organic solvents because it has a relatively high boiling point as well as a tendency to hang on with hydrogen-bonds whenever possible. Let's think about how residual water would affect our calculations.
If our precipitate is not completely dry when we measure the mass, we will think we have a higher mass of
than we actually do (since we are now measuring the mass of plus the residual water). A higher mass of will result in calculating more moles of in Krok , which will be converted into more moles of in our mixture. In the last step, we will end up calculating that the mass percent of is higher than it really is.
Lab tip: If you have time, one way to check for water in the sample is to recheck the mass a few times during the end of the drying process to make sure the mass is not changing even if you dry it longer. This is called drying to constant mass, and while it does not guarantee that your sample is completely dry, it certainly helps! You can also try stirring up your sample during the drying process to break up clumps and increase surface area. Make sure you don't tear holes in the filter paper, though!
: We forgot to balance the equation!
Remember how we said earlier that gravimetric analysis is really just another stoichiometry problem? That means that working from an unbalanced equation can mess up our calculations. For this scenario, we would be using stoichiometric coefficients from the following unbalanced equation:
This equation tells us (incorrectly!) that for every mole of
we make, we can infer that we started with mole of in the original mixture. When we use that stoichiometric ratio to calculate the mass of , we will get:
We just calculated that the mass of
in our mixture is double the correct amount! This will result in overestimating the mass percent of by a factor of :
: Adding in excess
In the last scenario we wonder what would happen if we didn't add
in excess. We know this would be bad because if is not in excess, we will have unreacted in solution. That means the mass of will no longer be a measure of the mass of in the original mixture since we won't be accounting for the still in solution. Therefore, we will underestimate the mass percent of in the original mixture.
A related and perhaps more important question we might want to answer is:
How do we make sure that we are adding
If we knew the answer to that question, we could be extra confident in our calculations! In this problem:
- We have
of a mixture that contains some percentage of .
- We also know from our balanced equation that for each mole of
, we will need moles of at a minimum.
It is okay if we have extra
, since once all the has reacted, the rest of the will simply stay part of the solution which we will be able to filter away.
If we don't know how many moles of
are in our original mixture, how do we calculate the number of moles of necessary to add? We know that the more moles of we have in our original mixture, the more moles of we need. Luckily, we have enough information to prepare for the worst case scenario, which is when our mixture is . This is the maximum amount of we can possibly have, which means this is when we will need the most .
Let's pretend that we have
. How many moles of will we need? This is another stoichiometry problem! We can calculate the number of moles of by converting the mass of the sample to moles of using the molecular weight, and then converting to the moles of using the molar ratio:
This result tells us that even if we don't know exactly how much
we have in our mixture, as long as we add at least we should be good to go!
Precipitation gravimetry is a gravimetric analysis technique that uses a precipitation reaction to calculate the amount or concentration of an ionic compound. For example, we could add a solution containing
to quantify the amount of a halide ion such as . Some useful tips for precipitation gravimetry experiments and calculations include:
- Double check stoichiometry and make sure equations are balanced.
- Make sure that the precipitate is dried to constant mass.
- Add an excess of the precipitating agent.