Chem 105A Lab Exam, Experiment 2 - Gravimetric Analysis, How much phosphorus in plant food?

What 3 nutrients are most likely to be lacking in soil?

Nitrogen, phosphorus, and potassium

What form is each nutrient present in?

Nitrogen - N

Phosphorus - P2O5

Potassium - K2O

What does the label 15-30-15 mean?

15% N

30% P2O5

15% K20

Is there any P2O5 in the plant food Any K20?

No, that's just how the P and K contents are quoted

What was the purpose of this lab?

Determine the amount of phosphorus in a fertilizer by the method of gravimetric analysis

What is gravimetric analysis based on?

Mass measurements

What information do we use to analyze the composition of the original sample?

Mass and composition (chemical formula)

What do we start with in this experiment and what do we do to it?

Start with a sample of water-soluble plant food

Precipitate the phosphorus in the form of magnesium ammonium phosphate hexahydrate

What type of substance is ammonia?

Weak base

Why must the hydroxide ion be provided in the form of a weak base?

If a strong base is used, Mg(OH)2 will precipitate instead of the desired MgNH4PO4*6H2O

What is also important to account for when precipitating solids other than acidity/basicity?

Temperature

How does temperature affect most solids?

Will become soluble at higher temperatures

Why did we need to keep MgNH4PO4*6H2O at room temperature?

At warmer temp, it will begin to lose water molecules and be converted to form the monohydrate

What was the point of creating MgNH4PO4*6H2O?

By determining the mass of that, we were able to determine the mass percent of phosphorus in the plant food

What was the mass % of P in P2O5?

43.64%

What was the mass % of P in MgNH4POr*6H2O?

12.62%

How should we be cautious around ammonia?

Do not inhale

Perform steps with it in fume hood

What is mass of P in product equal to?

Mass of P in original sample

What measurements did we need to obtain in this lab?

1. Mass of plant food

2. Mass of weight boat

3. Mass of weight boat + sample

4. Mass of sample

What mass of plant food did we aim to begin with?

About 1.0 gram

What did we do to the plant food sample first?

Dissolved it in a beaker with about 25 ml of DI water

What did we add to the dissolved plant food?

Scoop of activated carbon

Why did we add a scoop of activated carbon to the dissolved plant food?

To remove copper cations from solution

How do we know that we've added enough activated carbon?

The plant food solution becomes colorless

What do we do once the carbon and plant food solution becomes colorless?

Filter out all the solids using the Buchner funnel and vacuum filtration

What color should our filtrate be?

Clear and colorless

What does a blue filtrate indicate?

Not all of the copper ions were removed

What does a grey filtrate indicate?

Some of the carbon passed through the filter

What do we add to the filtrate? Why is MgNH4PO4*6H20 unlikely to precipitate at this point?

Add ~15 mL of 0.5 MgSO4

Too acidic for precipitate to form at this point

How do we raise the pH of the solution so the precipitate will form?

Add ~20 mL of ammonia (NH4OH) gradually while stirring

What do we do to the mixture once we add the ammonia?

Allow it to stand in the fume hood for 15 mins

Once removed from the fume hood, what do we do to the mixture?

Filter it to get the MgNH4PO4*6H2O white solid

What should we do if the precipitate is contaminated?

Wash with small amount of acetone and refilter

Do NOT use water (will dissolve solid)

How should we dry the solid as much as possible?

Acetone

How did we calculate the mass of P in the precipitate?

(mass of precipitate) x (mass % of P in precipitate found in pre lab)

Mass % P in precipitate = 12.62%

How did we calculate the mass of % P in plant food?

(mass P in precipitate) / (mass of plant food)

Steps for finding mass % of P in plant food?

1. Mass of p in precipitate

2. mass % of p in plant food

How did we find the mass % of P2O5 n the sample of plant food from the mass % of P in the sample of plant food?

1. Mass of P2O5 in sample

2. Mass % P2O5 in sample

What did I find for mass % P in plant food? For mass % P2O5 in plant food?

3.29%

7.49%

What as the actual composition of miracle grow?

24 - N

8 - P

16 - K

What could have made the mass of the precipitate too low?

Not all of it was scraped off the filter paper

What could have made the mass of the precipitate too high?

Carbon was not completely filtered out

What were we expected to find the percent phosphorus to be?

8%