Question

Pre-Lab Study Questions Name Gravimetric Analysis: The Determination of the % Phosphorus in Plant Food 1. The label on a plant food container reads 23-19-17. What, according to the label, is the minimum percentage of potassium in this plant food? What is the minimum percentage of phosphorus in this plant food? 2. MgNH.PO. - 6H2O has a solubility of 0.023 g/100 mL in water. Suppose a 5.02 g sample of precipitate were washed with 10 ml of water, similar to the amount that you will use to wash your precipitate. a) What number of grams of the MgNH.PO4 - 6H20 would be lost? b) What fraction of MgNH PO - 6H20 would be lost? 3. MgNH PO - 6H2O loses H2O stepwise as it is heated. Between 40°C and 60°C the monohydrate (MgNH.PO. H20) is formed and above 100°C the anhydrous material (MgNH,PO) is formed. Why, then, do the instructions say to air dry the precipitate and filter paper at room temperature?

Answer 1

1) The percentages are:

% K = 19%

% P = 17%

2) a) The lost mass is calculated:

m lost = 10 mL * (0.023 g / 100 mL) = 0.0023 g

b) The percentage is calculated:

% Lost = 0.0023 * 100 / 5.02 = 0.05%

4) The molar mass of the precipitate formed is 245.3 g / mol.

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