CHEM 101 / 105 HOUR EXAM II 7-OCT-03
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1 CHEM 101 / 105 HOUR EXAM II 7-OCT-03 NAME (please print clearly) / (signed) (last) (first) 1. a. Write a balanced chemical equation for the process of dissolving a quantity of solid sodium arsenate, Na 3 AsO 4, in water. Na 3 AsO 4( solid ) Na( + AsO4( b. Write a balanced chemical equation for the process of dissolving a quantity of solid magnesium nitrate, Mg(NO 3 ) 2, in water. Mg 2+ 1 ( NO3 ) 2( solid ) Mg( + 2 NO3( c. When present in solution, magnesium cations and arsenate anions form a precipitate, magnesium arsenate (FW = 350). Write a net ionic equation for the precipitation process Mg( + 2 AsO4( Mg3( AsO4 ) 2( solid ) d. How many grams of precipitate would be formed if ml of a M solution of sodium arsenate was mixed with ml of a 0.26 M solution of magnesium nitrate? m moles Mg 3 (AsO 4 ) 2 = m moles Na 3 AsO 4 1 mole Mg 3 2 mole Na ( AsO 3 4 AsO 4 ) 2? m grams = (0.095M )(45.28mL) grams
2 2. An organic compound is composed of carbon, hydrogen, and fluorine. The density of the compound in the gas phase is 2.78 g / L at 500 torr and 50 deg.c. The compound has been analyzed for it fluorine content, which is found to be 50.9 %. a. What is the molecular weight of the compound? g L atm atm ? MW K mole ( 1L) = ( K ) g / mole b. What is the molecular formula of the compound? mass fluorine in one mole = x 112 = 57 grams moles fluorine in one mole = 57 g F / 19 g / mole = 3 so the molecule contains THREE fluorine atoms. non-fluorine mass in compound = = 55 grams carbon and hydrogen. FOUR moles carbon has mass of 48 grams, so balance of 7 grams is hydrogen. (three carbon and nineteen hydrogen are possible, but less likely) C 4 H 7 F 3
3 3. a. Consider an aqueous solution of barium hydroxide (barium, at.no. 56). Write a balanced chemical equation showing the condition of this solute in solution. Ba ( OH ) 2 ( Ba( + 2 OH ( b. Consider an aqueous solution of hydrobromic acid, HBr. Write a balanced chemical equation showing the condition of this solute in solution. HBr 1+ 1 ( H ( + Br( c. Suppose the two above solutions are mixed. Write a net ionic equation appropriate for any ensuing reaction. H 1+ 1 ( + OH ( HOH d. Suppose ml of a M solution of hydrobromic acid is added to ml of a M solution of barium hydroxide. What is the final concentration of the ion from water present in an excess amount? H 1+ OH 1- HOH B4 m moles (23.46)(0.107)= (31.67)(0.040)(2)= lots react -1:-1: aftr none lots M = mmoles ml = ( ) [ OH 1- ] = 4.28 x 10 4
4 4. A gaseous organic compound is composed of carbon, hydrogen, and sulfur. Combustion of this compound is carried out so that volumes of individual reactants and products can be measured, all at same conditions of temperature and pressure. When three volumes of the compound undergoes combustion with oxygen gas, then nine volumes of carbon dioxide, twelve volumes of water vapor, and six volumes of monosulfur dioxide are formed. a. What is the molecular formula of the compound? When Pressure and Temperature are fixed, gas volumes directly relate to moles. cmpd CO 2 H 2 O SO 2 vols = moles by moles = vols molecules atoms/molecule 3C 8H 2S atoms/cmpd 3C 8H 2S ans C 3 H 8 S 2 b. What is the empirical formula of the compound? C 3 H 8 S 2 c. What volume of oxygen gas, at same conditions of temperature and pressure, is required for complete combustion of the original three volumes of compound? 9 volumes CO 2 contains 9 volumes O 2 12 volumes H 2 O 6 volumes O 2 6 volumes SO 2 6 volumes O = 21 volumes O 2
5 5. a. Consider an aqueous solution of ammonia. Write a balanced chemical equation showing the condition of this solute in solution ( 4( ( mostly very few NH + HOH NH + OH b. Consider an aqueous solution of sulfuric acid, H 2 SO 4. Write a balanced chemical equation showing the condition of this solute in solution. Assume all hydrogens are ionzied ( 4( all H SO 2 H + SO 2 4 none c. Suppose the two above solutions are mixed. Write a net ionic equation appropriate for any ensuing reaction ( ( 4( NH + H = NH d. Suppose ml of a M solution of ammonia is titrated against a M sulfuric acid solution. What volume of the acid will be required. Assume all hydrogens are ionized. mmoles H 2 SO 4 = mmoles NH mole H 1 mole NH 3 1 mole H SO 1+ 2 mole H 2 4 (? ml )(0.045 M) = (27.31 ml)(0.113 M) [ 1 / 1 ][ 1 / 2 ] vol = ml
6 6. A mixture of gases stored in a steel cylinder exerts a pressure of 750 torr. It was prepared according to the following recipe: grams of nitrogen gas, 50.0 grams of phosphine gas, PH 3, and grams of argon gas (at.no. 18). What is the partial pressure of phosphine gas in the mixture. nitrogen, N 2 phosphine, PH 3 argon, Ar given mass grams MW moles = (mass / MW) total moles mol fraction = (moles / total moles) partial pressure = (mole fraction)(p (total) ) 387 torr P ( PH3 ) = 387 torr
7 7. a. Balance the following half-cell reaction: 1 aq = 2 gas ClO( ) Cl ( ) ( acid) b. Balanced the following half-cell reaction: s = 3 8 aq Mn O ( ) Mn O ( ) ( acid) c. Write a balanced chemical equation for the reaction of the above two half-cells: d. What mass of Mn 2 O 3 would be required to completely react with ml of a M solution of sodium hypochlorite, NaClO, if the reaction had 80 % yield? m moles Mn 2 O 3 = m moles NaClO 3 mole Mn2O mole ClO? mgrams 158g / mole = ( M ) ( ml ) [ 1 / 4 ]? m grams = IF YIELD IS 100 %. BUT, yield is only 80 %, AND Mn 2 O 3 is a reactant, so MORE than 154 mg is needed. How much more? Mass required = 154 mg / 0.80 = 193 mg 193 mg, or grams
8 8. a. A mineral of chemical formula Fe Cr 2 O 4, reacts with carbon to form elemental iron, elemental chromium, and carbon monoxide. Write a balanced chemical equation for this process. FeCr 2 O C Fe + 2 Cr + 4 CO( gas ) b. What mass of the mineral would be required to form 5.00 L of carbon monoxide gas at 800 deg.c and 2 atm, if the reaction is 72 % efficient? moles FeCr 2 O 4 = moles CO 1 mole FeCr O 4 mole CO 2 4? grams ( 5L)( 2atm) 224 g / mole = L atm ( )( 1073K) K mole [ 1 / 4 ]? grams mineral = 6.36 grams, IF reaction were 100 % efficient. BUT, reaction is only 72 % efficient, AND FeCr 2 O 4 is a reactant, so more than 6.36 grams will be required. How much more? Mass required = 6.36 / 0.72 = 8.83 grams 8.83 grams
9 9. a. Balance the following half-cell reaction: aq = 4 2 aq S O ( ) SO ( ) ( base) b. Balance the following half-cell reaction: 2 6( = ( s) Pb( OH) PbO ( base) c. Write a balanced chemical equation for the reaction of the above two half-cells: d. What mass of PbO would be formed from reaction of ml of a M solution of Na 2 S 4 O 8, if the reaction were 60 % efficient? moles PbO = moles Na 2 S 4 O 8? m grams 223 g/mole 7 moles PbO 2 1 mole S4O 6 = ( ml )( M ) [ 7 / 1 ]? m grams = 11, 700; or, 11.7 grams, IF reaction is 100 % efficient. BUT reaction is only 60 % efficient, AND PbO is a product, so less will be formed. How much less? PbO formed = 11.7 x 0.60 = 7.02 grams grams
10 10. a. Suppose the average velocity of atoms of a certain gas was 275 m/sec at 25 deg.c. At what temperature will the velocity be reduced by one-third? New velocity = [ 275 ( 275 / 3 ) ] = 183 m / sec 2 =?T T = 110 K b. At some specified temperature suppose that the average velocity of neon atoms (at.no. 10) is 310 m/sec. At this same temperature, what would be the velocity of xenon atoms (at.no 54)? v v (xenon) (neon) = v (xenon) = 310 m / sec ( ) = 121 m / sec c. Suppose the time required for a certain number of carbon dioxide molecules to diffuse through a porous barrier is 87.3 seconds. If 127 seconds are required for an equal number of molecules of an unknown gas to diffuse through the same barrier, at the same temperature, what is the molecular weight of the unknown gas? 127? MW = g / mole? MW = 93.1 g / mole
CHEM 105 HOUR EXAM III 28-OCT-99. = -163 kj/mole determine H f 0 for Ni(CO) 4 (g) = -260 kj/mole determine H f 0 for Cr(CO) 6 (g)
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