AP Chemistry Unit 8-9

Chapters 8-9 Bonding and Structure

Hand In Lab 2 – Gravimetric Determination of Silver

“Gravimetric Determinations” are a standard procedure using weights of precipitated chemicals in the determination of the amount of a given component chemical within a mixture. Mineral Assay work is a common example of how this process has been applied. In our case we will be determining the percentage of Silver via precipitation with Chloride ions. We know from our work with Group 1 chemicals that Mercury, Silver and Lead all precipitate (become an insoluble salt) with the addition of a chloride (Cl^-1_(aq)) ion. What we will have to do is first make the Silver a soluble nitrate ion with treatment of Nitric Acid (making all metal soluble ions as nitrate salts) then precipitating ONLY the Silver with treatment of Hydrochloric Acid.

This process relies on a series of steps. First, the exact weight of the original or starting Silver sample must be known. Second, the sample for analysis must be brought into solution, Third, a selective reagent must be used that will precipitate the ion in question and Fourth, the precipitate must be filtered, washed and dried. As weight is a critical factor in these determinations only an Analytical scale can be used. They are the “box” scales in the corners of the room and they measure to the ten thousandth of a gram. Understandably, they are very expensive and delicate, so great care must be taken when measuring. Remember these rules –

Never weigh directly on the face of the scale. Use a weighing paper and

tare the scale before measuring

Have all scale doors closed as slight wind drafts can vary the weight of the

sample.

When the star (upper left corner) appears on the display, you may record

the weight of the sample.

The scale can only measure small sample weights so do not overload the

scale as it will damage the system.

Note: Wear goggles and aprons as we will be working with 2 different types of acid.

Procedure

1. Obtain a silver sample and determine its exact mass.

2. Place this sample in a clean 250 ml beaker and treat this sample with 20 ml of 6M HNO₃ to dissolve the ions. Additional acid with stirring may be needed to dissolve the ions. It is not necessary to record the total volume of acid used as it is not the limiting reactant.

3. Heat the 250 ml beaker in a water bath to a near but not boiling temperature and add 6M HCl until additional drops of HCl do not appear to cause any additional precipitation.

4. Obtain a sintered glass suction funnel, mark its ground glass face (white oval panel) with identifying marks from a pencil and determine its exact mass.

5. Turn on the sink outlet and place your finger/thumb over the vacuum port of the vacuum fitting to verify there is a gentle suction provided by the attachment. A greater velocity of water through the outlet creates a greater low pressure at the fitting.

6. With the 2 flask vacuum system and the suction fitting on the sink, turn on the water (to supply suction), insert the plastic fitting from the two flask system into the suction port (in the middle of the pipe attached to the sink outlet) and begin the filtering of the precipitate by pouring small amounts of the precipitate through the funnel (check fitting around the second flask to make sure they are not losing their fit to provide maximum pull through the filter).

7. Use a wash bottle and policeman to ensure you remove all the precipitate from the beaker. Be generous with the distilled water from the wash bottle to additional remove any adhered ions to the precipitate. You may have to shut down the suction and dump the side arm erlynmeyer flask at least once before you are finished washing the precipitate.

8. Remove the sintered glass funnel and place in the drying oven overnight (front right side of room) to remove the water and weigh the funnel on the following day.

9. To clean the funnel for later use, prepare a washing solution of 10 ml of 6M NH₃ in 200 ml of water. Tap filter gently against the edge of a garbage can to remove any loose precipitate and agitate gently with a rubber policeman to assist in loosening. To the remaining precipitate/filter system add the cleaning solution drop wise to the filter with suction until all the silver residue is removed (it will form a soluble ligand compound with the ammonia). Rinse and return the CLEAN filter for later use.

Data

Mass of sample-

Mass of clean empty funnel-

Mass of funnel with Silver Chloride-

Calculations

Mass of Silver Chloride-

Mass of Silver Nitrate-

Mass of Silver-

% Silver-

Questions

1. Write the reaction of silver with nitric acid (both ways-ionic and traditional)

2. Write the reaction of silver nitrate with hydrochloric acid (both ways)

1. Why is it expected to add slight excess Nitric and Hydrochloric acid?

2. What might happen if the Nitric Acid is in excess too much?