2005 D Dani Klingert

Answer the following questions that relate to laboratory observations and procedures.

(a) An unknown gas is one of three possible gases: nitrogen, hydrogen, or oxygen. For each of the three possibilities, describe the result expected when the gas is tested using a glowing splint (a wooden stick with one end that has been ignited and extinguished, but still contains hot, glowing, partially burned wood).

Glowing Splint Tests Nitrogen: When the glowing splint is inserted into the gas sample, the glowing splint will be extinguished.  It is extinguished because it blocks the possible oxygen gas from reaching the glowing splint and suffocates the stick. Hydrogen: When the glowing splint is inserted into the gas sample, a popping sound (explosion) can be heard. Oxygen: When the glowing splint is inserted into the gas sample, the splint will glow brighter or reignite.  This is because O 2 is needed for combustion reactions and when more oxygen gas is present, the combustion can occur.

For combustion reactions, a hydrocarbon or some form of carbon and oxygen gas must be present.

(b) The following three mixtures have been prepared: CaO plus water, SiO 2 plus water, and CO 2 plus water. For each mixture, predict whether the pH is less than 7, equal to 7, or greater than 7. Justify your answers.

Reactions with Water CaO (s) + H2O (l) → Ca(OH) 2(aq)  Ca(OH) 2 is an aqueous base (basic anhydride)  The pH of the solution will be greater than 7 because a base is formed. SiO 2(s) + H 2 O (l) → SiO 2(s) + H 2 O  No reaction-because SiO 2 is insoluble in water  The pH of the solution will be equal to 7 because SiO 2 is insoluble in water and doesn’t break up into ions. CO 2(g) + H 2 O (l) → H 2 CO 3(aq)  H 2 CO 3 is an aqueous acid (acid anhydride)  The pH of the solution will be less than 7 because an acid is formed

Definitions Acid Anhydride- a substance that combines with water to form an acid. Generally, oxides of nonmetals are acid anhydrides. Basic Anhydride- a substance that combines with water to form a base. Generally, oxides of metals are basic anhydrides.

pH Values Acids have a pH under 7 Neutral solutions have a pH of 7 Basic solutions have a pH greater than 7.

(c) Each of three beakers contains a 0.1M solution of one of the following solutes: potassium chloride, silver nitrate, or sodium sulfide. The three beakers are labeled randomly as solution1, solution 2, and solution 3. Shown below is partially completed table of observations made of the results of combining small amounts of different pairs of the solutions. (i) Write the chemical formula of the black precipitate. (ii) Describe the expected results of mixing solution 1 with solution 3. (iii) Identify each of the solutions 1, 2 and 3. Solution 1Solution 2Solution 3 Solution 1 black precipitate Solution 2 no reaction Solution 3

Possible Reactions: AgNO 3(aq) + Na 2 S (aq) → Ag 2 S (s) + NaNO 3(aq)  Ag + (aq) + NO 3 - (aq) + 2Na + (aq) + S 2- (aq) → Ag 2 S (s) + Na + (aq) + NO 3 - (aq)  Ag 2 S (s) - black precipitate KCl (aq) + AgNO 3(aq) → KNO 3(aq) + AgCl (s)  K + (aq) + Cl - (aq) + Ag + (aq) + NO 3 - (aq) → AgCl (s) + K + (aq) + NO 3(aq)  AgCl (s) - white precipitate Na 2 S (aq) + KCl (aq) → NaCl (aq) + K 2 S (aq)  2Na + (aq) + S 2- (aq) + K + (aq) + Cl - (aq) → Na + (aq) + Cl - (aq) + 2K + (aq) + S 2- (aq)  no reaction- all break up into ions and nothing new forms

So from the table we know that solution 1 and 2 have to be either AgNO 3 or Na 2 S so that a black precipitate can be formed. We also know that solution 2 and 3 have to be either Na 2 S and KCl. Since Na 2 S is repeated twice, we know it has to be solution 2. Then from that we know to form the black precipitate, solution 1 must be AgNO 3 and solution 3 is KCl. Solution 1Solution 2Solution 3 Solution 1 black precipitate white precipitate Solution 2 black precipitate no reaction Solution 3 white precipitate no reaction

Answers to Part C The chemical formula of the black precipitate formed by solutions 1 and 2 is Ag 2 S. A white precipitate will be produced when the solutions 1 and 3 are mixed. The chemical formula for this would be AgCl. The identities of the solutions are: solution 1 is silver nitrate (AgNO 3 ), solution 2 is sodium sulfide (Ag 2 S), solution 3 is potassium chloride (KCl).