What is the solubility of potassium nitrate in 5 °c water

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The various methods of separating mixtures find a wide range of applications in industry. This particular task involves analysing and using solubility data to see how salts can be separated by crystallisation.

Task 1: Producing a graph to represent solubility data

1. Many substances are soluble in water. Some, like sodium chloride, dissolve to form a clear solution, others dissolve to form coloured solutions. Not all compounds have the same solubility. The solubility of a substance is usually given in g/100cm3. At room temperature (25 °C) sodium chloride has a solubility of 36 g / 100 cm3 - this means that 36 grams of sodium chloride will dissolve in 100 cm3 of water.
   When no more solid can be dissolved in a solution, the solution is said to be saturated.
2. Open Yenka file Model 1. Select each of the beakers in turn.  Select and look at their atom viewers and describe each of them.
3. Open Yenka file Model 2. It shows two dishes of potassium nitrate and two beakers of water. Add the potassium nitrate to each beaker, and then place one of them on the heater. What differences do you notice between the two beakers?
4. Is potassium nitrate more soluble in warm or cold water?
5. Some solubility data for two salts is presented in the table below:

   	Potassium nitrate (KNO3)	Sodium nitrate (NaNO3)
   Temp. °C	g / 100 g water	g / 100 g water
   0	14	73
   10	21	81
   20	32	88
   30	45	96
   40	63	105
   50	84	114
   60	108	124
   70	136	134
   80	168	148
   90	203	161
   100	245	178

6. On your graph paper, plot a graph of the solubility of potassium nitrate and sodium nitrate in g/100g water (from 0 to 260 on y-axis) versus temperature °C (from 0 to 100 on x-axis). Try to join up the points with the 'best curve' to improve accuracy when reading data from the graph.

   Answer

   
   
   Graph

Summary

The solubility of a salt can be accurately described using a graph. In addition to plotting it accurately, and using the best line or curve, it is important to make sure that the correct units are clearly shown on all data tables and graph axes.

Task 2: Interpreting and using the solubility graph

1. Use the graph you have drawn to answer the following questions.
2. (a) What is the general pattern shown by the graph? Is it what you would expect? (b) What is the solubility of potassium nitrate at 45 °C? (c) At what temperature is the solubility of sodium nitrate 90 g / 100 g water?

   (d) At what temperature do the two salts have the same solubility?

   Answer

   (a) Most salts show a clear trend of increasing solubility with increased temperature i.e. more salt dissolves in the same amount of water at a higher temperature. (b) About 72g/100g water (71-73 ok) (c) 23°C (22-24 ok)

   (d) Where the graph lines cross at 68°C (67-69 ok)

3. (a) How much potassium nitrate will dissolve in 500g of water at 76 °C?
   (b) How much sodium nitrate will dissolve in 10g of water at 55 °C?

Summary

From a basic graph of data you can read of the solubility at any temperature within the data limits. You can do simple proportion calculations for any mass of salt or water to calculate how much is dissolved based on g salt / 100 g of water.

Task 3: Using the solubility graph to decide on crystallisation conditions

1. A mineral ore salt deposit consists of 50 % sodium nitrate and 50 % potassium nitrate. The potassium nitrate is more valuable for fertiliser salts than the sodium nitrate. It would be ideal to crystallise out the potassium nitrate and sodium nitrate separately.
   In a laboratory simulation experiment, 200 g of the ore (100 g of each salt) was dissolved in 100 g of hot water at 90 °C. The solution was allowed to cool and the salt crystallised was collected and analysed. However, it is possible to use our graph to predict what might happen (in industry, a computer model would be used).
2. (a) At what temperature will salt crystals first appear, and which salt will it be? (b) At what temperature will the other salt begin to crystallise? (c) Is it possible to crystallise pure potassium nitrate? If so, suggest the experimental conditions needed. (d) What mass of potassium nitrate will have crystallised out when the mixture has cooled to 50 °C? (e) Why is it so easy to separate the salts?

   (f) If you weighed the remaining solution and all the crystallised salts, what would be the total mass and why?

   Answer

   (a) Potassium nitrate will begin to crystallise at about 56 °C, when its solubility is 100 g/100g water. (b) Sodium nitrate (at about 34 °C), when its solubility is 100g/100g water (c) Yes. If you collect the crystals forming at over 34 °C and below 56 °C. Above 56 °C nothing crystallises for this mixture, and below 34 °C both salts form crystals at the same time. (d) Initially there is 100 g of potassium nitrate in solution, at 50 °C the solubility is 84 g/100g (original data or graph), so 16 g will have crystallised out. (e) They are not chemically combined, just a physical mixture.

   (f) A total of 300 g (100 g water + 200 g of salts). Mass is always conserved however much it changes its form.

Summary

Once you have a database of solubility (or other data) you can make accurate predictions without having to do lots of experiments. Let the computer take the strain!

Teacher Summary

• This is not an easy graph to draw, and may tax students.
• If students have difficulties, then MS Excel could be used to help draw the graph.
• Task 3 is very demanding, but could be useful for more advanced students.

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Solubility is the mass of a solute that can be dissolved in a given mass of solvent.

Solubility varies according to temperature for the same solute and solvent.

The solubility graph for potassium nitrate in water is shown below.

EXAMPLE

What is the solubility of potassium nitrate at 50 °C?

Solution

Read along the horizontal axis until you reach 50 °C. From there, extend a line upwards until it meets the graph.

Then extend a horizontal line to the vertical axis. Read the temperature at that point.

The solubility is about 90 g of KNO₃ per 100 g of water.