Solubility of Ammonia

Introduction

Ammonia, NH₃, is a gas at room temperature but you may be familiar with it as a water solution which is commonly used as a cleaning agent. In this experiment you will generate ammonia gas and investigate its solubility in water.

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1. Pick up two Beral pipets, one long and one short; a 100-mL beaker; and a 250-mL beaker. (Notice that the longer pipet has a slightly narrowed end so that it will fit into the end of the smaller pipet - try this out. The longer pipet will be your gas generator and the smaller will be the collecting tube.)
2. Fill the 100-mL beaker with cold water and set it aside. Fill the 250-mL beaker with water and heat it until bubbles are visible on the inside of the beaker. You want the water hot but not boiling.
3. Take your longer pipet and go to the hood where your teacher will have a small beaker of concentrated ammonia solution (variously labeled NH₃(aq) or NH₄OH). Squeeze the pipet to push out the air and place the tip into the ammonium hydroxide to draw up enough to fill the pipet about half full.
4. At your lab desk, set the shorter pipet onto the longer pipet so that the elongated tip of the longer pipet extends into the bulb of the shorter pipet.
5. Place the longer pipet in the hot water bath to warm the ammonium hydroxide. (As the ammonium hydroxide is warmed, NH₃ is released. Vigorous bubbling of the concentrated ammonia will be observed.) Pump the smaller pipet several times to fill it with the gas. Remove the longer pipet from the hot water bath and detach the smaller pipet.
6. Observe the odor of the gas collected in the small pipet by wafting a small amount of it from the tip of the pipet towards your nose. (Describe the characteristic odor of ammonia.)
7. Place the tip of your short pipet into the cold water in the small beaker. Squeeze gently to push out one bubble of ammonia which will allow a small amount of water to enter the tube. What do you observe? (What does this tell you about the solubility of ammonia gas in water?)
8. Empty your small pipet completely, being sure to shake out all the water, and refill it with ammonia gas from your generator. You will not need to add more ammonium hydroxide, just rewarm the generator. On this trial, add an acid/base indicator to the cold water in the beaker. What does this show you about the acid/base character of the ammonia in water solution?
   

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1. You have prepared ammonia gas by heating a water solution of ammonia. This aqueous solution is sometimes written NH₃(aq) and sometimes written NH₄OH. Using your observations from this experiment discuss why both of these are useful in understanding the behavior of ammonia gas when it is dissolved in water.
2. Why do commercial cleaners which contain ammonia as the active ingredient include cautions such as: "do not breath the vapors" and "keep off of skin"?

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• Wear your safety glasses at all times.
• Do not inhale ammonia gas. Use adequate ventilation.

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1. This represents the dual equilibrium which appears to exist when NH₃ dissolves in water. One one hand, we may regard it simply as hydrated NH₃ molecules, evidenced by the fact that NH₃ gas is easily driven off when it is heated.

   NH₃(aq) --> NH₃(g) + H₂O

   On the other hand, when one tests the water solution of NH₃ a weak base is indicated.

   NH₃(g) + H₂O --> NH₄OH

   NH₄OH --> NH₄⁺ + OH^-

   Ammonia gas is highly soluble in water.

2. Commercial cleaners containing ammonia release enough NH₃ gas even at room temperature so that breathing the vapors is very dangerous since the ammonia gas will dissolve in the moisture of the mucous membrane and the lungs. As with any base, it is also a skin irritant and should be avoided or washed off with a lot of water.

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• "long" Beral pipet (pull out the end slightly to form a taper and cut so that the student will have a pipet with a tapered tip as shown below)
• "short" Beral pipet (cut these at an angle so that they will have a tip about 1 cm in length as shown below)

  

• 100-mL beaker
• 250-mL beaker
• Bunsen burners or hot plate
• 20 mL of concentrated NH₃ in a small beaker under the hood (keep this covered with a watch glass when the students are not filling their pipets)
• Beral pipets of indicator (phenolphthalein works well). The teacher may want to fill these pipets prior to the class.

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The Woodrow Wilson microscale version of this lesson was developed by:

Dianne N. Epp
East High School
1000 S. 70th
Lincoln, NE 68510

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