A horticulture supplier prepares potassium nitrate crystals for a vertical farm. The compound is used in a nutrient solution because it provides potassium a...
Question 1 Report
A horticulture supplier prepares potassium nitrate crystals for a vertical farm. The compound is used in a nutrient solution because it provides potassium and nitrogen. A technician dissolves a weighed sample in hot water, then cools the solution to check how many crystals can be recovered. Fig. 1 shows the filtration stage after the crystals have formed. The funnel paper holds the crystals, while the liquid that passes into the flask is the filtrate.
The sample contains 18.0 g of potassium nitrate, KNO3, dissolved in 50.0 g of water at 70 °C. At 70 °C, 31.0 g of potassium nitrate dissolves in 100 g of water. At 20 °C, only 13.0 g dissolves in 100 g of water.
(a) Use the solubility data to calculate the maximum mass of potassium nitrate that remains dissolved at 20 °C and the mass of crystals that can be collected. [3]
(b) Describe how the technician should obtain dry crystals after cooling the solution. [3]
(c) Complete the ionic equation, including state symbols, for potassium nitrate dissolving in water.
KNO3(s) → ______ + ______ [2]
(d) Suggest why distilled water, rather than nutrient solution containing other compounds, is used for this check. [1]
Answer Details
(a) At \(20\degree\text{C}\), \(13.0\text{ g}\) dissolves in \(100\text{ g}\) of water. For \(50.0\text{ g}\) of water:
\[\frac{13.0}{100}\times50.0=6.50\text{ g}\]
So \(6.50\text{ g}\) remains dissolved. The crystals collected are:
\[18.0\text{ g}-6.50\text{ g}=11.5\text{ g}\]
[3]
(b) Filter the cooled mixture to separate the crystals from the filtrate. Rinse the crystals with a small amount of cold distilled water, then dry them between filter papers or in a warm drying oven. Cold water is used for rinsing to avoid dissolving much of the product. [3]
(c) Potassium nitrate is ionic, so it separates into its ions in water:
\[\text{KNO}_3(s)\rightarrow\text{K}^+(aq)+\text{NO}_3^-(aq)\]
[2]
(d) Distilled water avoids other dissolved compounds contaminating the crystals or changing the solubility and mass measured. [1]
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