1) An element E is a constituent of three volatile hydrogen compounds for which the
following data are available:
Compound
Density of gas,
% E by Mass
referred to S.T.P.
A
1.24 grams/liter
78.42
B
2.39 grams/liter
81.33
C
2.84 grams/liter
85.82
a) Find the aproximate atomic weight of E.
b) Determine the formula of compound A.
c) Determine a more precise atomic weight of E with reference to the atomic weight of
hydrogen as 1.008.
2)
a) A student studied the reactions which occur when magnesium metal is heated in air as
follows. A sample of granulated magnesium weighing 20.0 grams was heated for some
time in a current of air passing over a loosely covered crucible. The sample was then
allowed to cool to room temperature. The powdery gray material was then boiled with
excess water, and the ammonia gas given off was caught in 2.00 x 102 milliliters of 1.00molar sulfuric acid. Analysis showed that 83.3 milliliters of the acid had been neutralized
by the ammonia to form (NH4)2SO4. Calculate the weight of magnesium in the original
sample which had been converted to magnesium nitride.
b) The residue remaining after the treatment with hot water, described in (a), was
dissolved in excess 3.00-molar hydrochloric acid and was found to displace 2.80 x 103
milliliters of hydrogen gas, measured over water at 26 °C and at a barometric pressure of
757 milliliters Hg. Calculate the weight of metallic magnesium remaining from the
original sample after it had been heated in air.
3) A solution contains 0.10 mole of Cl¯ ion and 0.0010 mole of CrO42¯ ion per liter. Ag+
ion in the form of solid AgNO3 is added to this solution. Solubility product constants:
AgCl 1.8 x 10¯10; Ag2CrO4 2.4 x 10¯12
a) Calculate the concentration of Ag+ ion necessary to start the precipitation of AgCl.
b) Calculate the concentration of Ag+ ion necessary to start the precipitation of Ag2CrO4.
c) What is the composition of the first permanent precipitate formed when the AgNO3 is
added to the above solution?
d) What fraction of the Cl¯ ion originally present remains in solution when Ag2CrO4 first
begins to precipitate?
4) The numerical value of the equilibriom constant at 425 °C for the reaction H2(g) +
I2(g) <===> 2HI(g) has been found to be 54.5, expressed in terms of molar
concentrations. If 0.30 mole hydrogen and 0.30 mole iodine are introduced into an
evacuated chamber having a volume of 5.0 liters and the system is held at 425 °C, find
the concentrations in moles per liter of H2, I2, and HI at equilibrium.
5) Chlorine gas is bubbled through a cold solution of sodium hydroxide.
6) Acidified potassium permanganate solution is added to a dilute solution of potassium
sulfite.
7) Sodium hydroxide solution in excess is added to a solution of magnesium nitrate.
8) A mixture of 1-molar hydrochloric acid and 1-molar sodium bromide is warmed.
9) Ammonia gas is dissolved in 6-molar soduim hydroxide solution.
10) Solid radium oxide is added to water.
11) Tetraborane, B4H10, is burned completely in an excess of oxygen.
12) Solid silver chloride is treated with concentrated (15-molar) ammonia solution.
13) Give a brief explanation, chemical, physical, or both, as to why the dissolution of
some salts in water is an endothermic process.
14) A mixture of hydrogen and chlorine gases is stable when stored in the dark but is
explosive when exposed to sunlight. Explain this difference in the speeds of reaction.
15) A 0.1-molar aqueous solution of aluminum sulfate has a pH of around 3. Explain this
observation in terms of the Arrhenius theory and of the Bronsted-Lowry theory of acids
and bases.
16) Gaseous hydrogen bromide can be prepared by the action of concentrated phosphoric
acid (H3PO4) on solid potassiom bromide. If concentrated sulfurid acid is used, a very
impure product is obtained. If either dilute phosphoric acid or dilute sulfuric acid is used,
no hydrogen bromide is evolved. Explain these observations.
17) How many isomers are there of the compound whose molecular formula is C2H2Cl2?
Describe how the various isomers differ from one another.
18) Describe at least three methods of determining molecular weights. The principle upon
which each of the methods is based should be different. These principles should be
included in the descriptions.
19) The forces between atoms, ions, and molecules vary greatly in nature and magnitude.
Discuss as many kinds of these forces as you can. Include references to specific
substances.