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## Group Activity: Graphing Solubility

### Introduction

Data in tables can be extremely useful and informative. Sometimes, though, making a graph of data makes relationships between variables much more clear than a table can. In this activity you will graph four sets of data on the same sheet of graph paper. These data describe the maximum solubility of four different substances in water as a function of temperature. Two of the substances are gases and two of the substances are solids.

Maximum solubility is also known as saturation. When a solution is saturated it means that the solvent has dissolved as much of the solute as it can and no more solute will dissolve. If more solute is added to a saturated solution it will not dissolve. If the solute is a solid it will remain visible in the solid phase. On the level of ordinary observation nothing appears to be happening. On the molecular level, particles of solid are constantly dissolving and exiting the solution to become solid again.

#### Data

Here are the data you will graph:

 Temperature (°C) Grams of NaCl Table Salt solid Grams of O2 Oxygen gas Grams of NH3 Ammonia gas Grams of C12H22O11 Table Sugar (sucrose) solid 0 34.5 14.0 92.2 64.4 10 35.0 11.0 72.1 65.4 20 35.4 8.8 57.0 66.7 30 35.8 7.5 45.9 68.3 40 36.2 6.5 37.8 70.1 50 36.6 5.7 29.6 72.1 60 37.0 5.0 24.5 74.2 70 37.4 4.6 19.4 76.5 80 37.8 4.0 15.3 78.7 90 38.2 3.5 10.2 80.7 100 38.6 3.1 8.1 82.6

The table shows the number of grams of solute that will combine with 100 g of water to make a saturated solution of that solute. For example, 34.5 g of NaCl plus 100.0 g of water make up 134.5 g of solution and this would be the maximum amount that would dissolve at 0°C.

The numbers in the table can be manipulated to find out how much solute you would add to any amount of water to make a saturated solution. Volumes of water in mL can stand for mass of water since for low temperatures because the density of water is almost exactly 1 g/mL. Say you want to make a saturated solution with 50 g of water at 20°C:
 ```Set up a proportion like this: 35.4 g salt x g salt -------------- = ---------------- 100 g water 50 g water ``` ```this simplifies to: 50 ------ × 35.4 g salt = 17.7 g 100 ```

So, to prepare the solution described you would mix 17.7 g of salt with 50.0 g of water. The total mass of the solution you make would be 67.7 g. The table relates how much of each solute will dissolve in 100 g water at each temperature. For other amounts of water you just set up a simple proportion.

Say you want to make 100 g of a saturated solution. From the table you can read that 100 g of water requires 35.4 g of salt to be saturated at 20°C. This makes a total solution mass of 135.4 g. That means that 135.4 g of solution contain 100 g water and 35.4 g salt. This is more than you want to make. In order to make a desired total mass for the solution you need to find the percent solute by mass. To calculate percentage by mass just divide and multiply by 100%:

```    35.4 g salt
------------------ × 100% = 26.14%
135.4 g solution
```

So the salt solution is 26.14% salt by mass. The other 73.86% of the solution’s mass is water. Now it’s easy to make 100 g of solution. Just measure 26.14 g of salt and add water until the total mass is 100 g. To make 50 g total mass for a saturated solution at 20°C just measure 0.2614(50) = 13.07 g of salt and add water until the total mass is 50 g.

1. Temperature belongs on the x-axis and grams of solute belongs on the y-axis. All four substances should be graphed on the same side of the same sheet of graph paper.
2. Make an easy-to-read graph that fills the space you make available for it (no blank spaces).
• This is best accomplished by setting up your axes before you start graphing.
• Make sure the largest value to be graphed on each axis will be on the graph.
• Select a spacing on the available scale so that you can easily figure out where each decimal is located.
3. Connect the dots for each substance and make each substance have a different color on the graph. Label which color goes with which substance.

#### Questions

Answer these questions on a separate sheet of paper using complete sentences. Show work for all mathematical questions.

1. Can gases be dissolved in water? What evidence (besides the given data) do you have that gases do dissolve in water? Provide at least two examples from your own experience.
2. Which materials increase their solubility as temperature increases? Which materials decrease their solubility?
3. In general, based on the data you have, how does the solubility of a gas in water change as temperature increases?
4. In general, based on the data you have, how does the solubility of a solid in water change as temperature increases?
5. In a solution of ammonia at 0°C is there more water present or more ammonia? Justify your answer using the data or your graph.
6. Calculate the percentage of ammonia by mass in a saturated solution at 0°C. The table shows that 92.2 g of ammonia will saturate 100 g of water, which makes for a total mass of 192.2 g.
7. How much oxygen (in grams) dissolves in 100 g of water at 10°C? How much would dissolve in 1000 g water?
8. How much oxygen (in grams) dissolves in 100 g of water at 40°C? How much would dissolve in 500 g water?
9. What is the percentage of oxygen by mass in water at 20°C?
10. Aquatic life requires oxygen to live, just as terrestrial life does. Fish and invertebrates tend to be much more abundant in polar waters than in tropical waters. Given the data you have studied in this activity, explain why this is so.
11. Carbon dioxide is dissolved in water to make soft drinks. Carbon dioxide will dissolve different amounts in water at different temperatures. Which of the curves you drew do you think carbon dioxide will most be like? Explain.
12. People enjoy drinking carbonated soft drinks chilled from the refrigerator or with ice. Is there a sound chemical reason for keeping soft drinks cold? What is it?
13. A little more salt (NaCl) dissolves at high temperature versus low temperature. Is the difference significant? Compare the percentage of salt by mass at 20°C to the percentage at 90°C to come up with an answer.
14. Do calculations to figure out how to make a saturated solution of table sugar at 20°C with a total solution mass of 30 g. How much sugar would you use? How much water? Hint: figure out the percentage by mass of sugar in a saturated solution at 20°C and use that information to calculate the amount of sugar. There is an example of this type of calculation in the text.