Analytical Gravimetric Determination: Percentage of
Phosphate in Plant Food
Introduction:
Gravitational analysis experiments aim to determine the amount of a constituent by
measuring masses. Normally, precipitation reactions will play a role in the procedure. The basic
steps taken to determine the quantity of an element in a sample are as follows: measure the total
mass of the sample, dissolve the sample in distilled water, mix with a solution that contains an
excess of a species that will react with the element in question to form a precipitate, and measure
the mass of the precipitate formed. The quantity of the sample’s mass that is allocated with the
element in question can be determined by performing stoichiometric calculations using the molar
mass of the precipitate.
In this experiment you will be determining the percentage of phosphorous in a sample of plant
food. The basic nutrients that plants receive from plant food are nitrogen, phosphorus, and
potassium. The phosphorus component of plant food is contained in the diphosphorus pentoxide,
P2O5, compound. Determining the exact amount of phosphorus in a sample of plant food must
be done experimentally, as the diphosphorus pentoxide is mixed with ammonium nitrate, urea,
and potassium oxide. Diphosphorus pentoxide reacts with water within the oh range of our
solution according to the equation below.
P2O5(s) + 3H2O(l)  2HPO4-2(aq) + 4H+(aq)
When magnesium sulfate and ammonia are added to the solution the following precipitation
reaction will take place.
Mg2+(aq) + NH3(aq) + HPO4-2(aq) + 6H2O(l)  MgNH4PO4*6H2O(s)
We will call this reaction a precipitation reaction because the product is a solid that does not
dissolve in water, and thus, precipitates from the solution. The solution can then be filtered to
collect the precipitate. After allowing the precipitate to dry, its mass can be measured and
calculations can be performed to find the percentage of phosphorus by mass in your sample of
plant food.
Purpose:
To determine the mass percent of phosphorus in a sample of plant food through
gravimetric analysis.
Material:

Balance

Two vacuum
filter paper

Solid plant
food

0.4 M magnesium
sulfate solution

Stirring Rod

Wash bottle
with distilled
water

100 mL
graduated
cylinder

3% or 2.0M
ammonia solution

Rubber
police men

Buchner
Funnel

10 mL
graduated
cylinder

Phenolphthalein
solution

150 mL
beaker

Vacuum
filtration
apparatus

Two 250
mL beaker
Procedures:
1. Use the balance to measure about 3.0 g of plant food in a clean and dry 150 mL beaker to
the nearest 0.001 g.
2. Add approximately 40 mL of distilled water to the beaker and stir the contents for three
minutes. Some of the solid material in the plant food may not dissolve. When you are
ready to stop stirring, rinse the section of the stir stick that was in the solution with
distilled water using the water bottle. This will carry any ions on its surface back into the
solution. If there is any undissolved material in the beaker you will need to filter the
solution to get rid of it. If there is no solid material in the beaker you do not need to filter
at this time, but you will need to filter the precipitate later in the experiment.
3. Set up the vacuum filtration apparatus as your teacher shows you. Make sure you obtain a
piece of vacuum filter paper.
4. Decant the solution by placing the glass stirring rod over the diameter of the beaker so
that one end rests in its spout and about 3.5 cm of the stirring rod protrudes passed the
edge of the spout. Pick up the beaker with one hand so that your hand is on the opposite
side from the spout and your hand is under the stir stick and you are holding the stirring
rod with your index finger. When you start pouring the liquid will run down the stirring
rod to its tip. This effectively provides you with the control needed to prevent spills and
to drop the liquid directly onto the filter paper. The stick also acts as a blocker that keeps
most of the solid in the beaker. Make sure not to over fill the funnel.
5. After all of the liquid has run through the filter, sweep up any solid in the beaker with the
rubber policeman and brush it onto the filter paper. Rinse the sides and bottom of the
beaker with distilled water from your wash bottle and pour the rinse water through the
filter. This will wash away any remaining solid and pull the ions from the sides of the
beaker into the solution into the flask.
6. After the liquid has finished running through the filter, rinse the filter paper and its
contents with distilled water from the water bottle one more time to wash any ions into
the solution into the flask
7. When the liquid has run through the filter, remove the filter paper and its contents and
deposit them in the garbage can.
8. Transfer the contents of the flask to a 250 mL beaker and rinse the flask thoroughly to
ensure that all of the ions are transferred to the beaker.
9. Add 45.0 mL of 0.4 M MgSO4 to the solution in the beaker.
10. Add 2 to 3 mL of 3% ammonia solution at a time to the solution in the 250 mL beaker.
Stir the solution after each addition of ammonia solution.
11. Repeat step 10 until the white precipitate ceases to form or until the solution turns any
shade of purple. Do not continue adding the ammonia solution after the solution has
turned any shade of purple! You will likely add about 150 mL of ammonia solution, but
you must do it slowly.
12. Let the solution sit undisturbed for 15 minutes to allow all of the precipitate to form and
settle on the bottom of the beaker.
13. Set up the vacuum filtration apparatus as the teacher showed you.
14. Write your name on a weigh boat and record the weight of the weigh boat and a piece of
vacuum filter paper
15. Decant the solution. Keep as much precipitate as possible in the beaker so you do not
clog the filter paper
16. After all of the liquid has run through the filter, sweep up any solid in the beaker with the
rubber policeman, and brush it onto the filter paper. Rinse the sides and bottom of the
beaker with distilled water from your wash bottle, and pour the rinse water through the
filter. This will wash any remaining solid onto the filter paper.
17. After the liquid has finished running through the filter, rinse the filter paper and its
contents with distilled water from the wash bottle one more time to wash any ions into
the waste beaker below.
18. When the excess liquid has run through the filter, carefully remove the filter paper and
the precipitate, and place it in a clean beaker to dry.
19. Store the beaker containing the filter paper and precipitate in a safe place and precipitate.
By subtracting the mass of the filter paper from this mass, you will obtain the mass of the
dry precipitate.
20. Clean all glassware, and wash your hands
Pre-Lab Questions:
1. Suppose you do not filter out all of the solid material that does not dissolve in the
beaker in step 2.
a. How would this affect your calculated value for the percent composition by
mass of phosphorus in your sample? Justify your answer
b. How would this affect your calculated value for the percent composition by
mass of phosphorus in your sample? Justify you answer.
2. What are the benefits of decanting a solution?
3. Why is it important that you only add a small volume of solution to the filtering
apparatus at a time?
Data:
Mass of Empty Beaker (g)
Mass and Beaker and Plant Food (g)
Mass of Plant Food Sample (g)
Mass of Filter paper (g)
Mass of Dry Filter Paper and Precipitate (g)
Mass of Precipitate (g)
Calculations:
1.
2.
3.
4.
5.
6.
Calculate the molar mass of MgNH4PO4*6H2O
Calculate the mass of phosphorus in your sample
Calculate the mass % composition of phosphorus in your sample
Calculate the number of phosphorus atoms in your sample.
Calculate the mass of P2O5 in your sample.
Calculate the percent composition by mass of diphosphorus pentoxide in your
sample?
7. Calculate the number of diphosphorus pentoxide molecules in your sample.
8. Calculate the percent composition by mass of water in MgNH4PO4*6H2O
Analysis:
1. Suppose your sample of solid MgNH4PO4*6H2O was not completely dry when you
measured its final mass. How would this affect your calculated value for the mass %
composition of phosphorus in your sample? Explain.
2. Suppose a student did not rinse the filter paper with distilled water one last time before
allowing it to dry (step 20). How would this affect the student’s measurements and
calculations for the percent composition by mass of diphosphorus pentoxide? Justify your
answer.
3. Obtain the percentage composition by mass of diphosphorus pentoxide that is advertised
on package. Does it appear as through the manufacturer added more, less, or about the
same amount of diphosphorus pentoxide as they advertised?