Unit 1 & 2
The molar mass of sulfur is 32.06 g/mol
Calculate the mass in grams of a sample sulfur of containing 2.01 x 1024 atoms.
107 grams
Explain: We can use Avogadro's number to convert from representative particles (such as atoms, molecules, or formula units) to moles of a substance. Then, we can use the substance's molar mass to convert from moles to grams.
What are the intermolecular forces that can be identified in benzene?
London Dispersion Forces
Explain: Benzene is nonpolar and made up of only carbon and hydrogen.
The rate law for a particular reaction is rate = k[X]2. In an experiment, the initial rate of the reaction is determined to be 0.080 mol/(L x s) when the initial concentration of X is 0.20 mol/L.
What is the value of the rate constant, k, for the reaction?
2.0 L /(mol x s)
Explain: To find the value of the rate constant for the reaction, solve the rate law for k. Next, plug in the initial rate and concentration given in the text.
MgCO3 (s) <--> MgO (s) + CO2 (g)
What is the expression for the reaction quotient, Qc, for the reaction represented above?
Qc = [CO2]
Explain: MgCO3 and MgO are both solids so they are exempt from the equation.
Br2 (l) --> Br2 (g)
List the signs of H and S for the process represented above at 58.8 C and 1 atm? ( The normal boiling point of Br2 (l) is 58.8 C.)
H and S are positive
Explain: H is positive because the process is endothermic. The entropy increases, so S is positive.
Which of the following electron configurations represents the element with the highest electronegativity?
A) 1s22s22p2
B) 1s22s22p6
C) 1s22s22p63s2
D) 1s22s22p63s23p2
B) 1s22s22p6
Explain: Generally, electronegativity increases as you move from left to right across a period and decreases as you move down a group. As a result, the most electronegative elements are found in the top right of the periodic table. In this case, the element closest to the top right of the table is Oxygen.
List the substances HCN, N2, and HF in order of increasing intermolecular forces.
N2, HCN, HF
Explain: N2 only has LDF because it is diatomic, HCN has dipole-dipole forces, and HF is hydrogen bonded because it is compromised of a hydrogen and fluorine atom.
Explain why an increase in reactant concentration generally increases the rate of a chemical reaction?
The collision rate between reactant particles is increased.
Explain: The rate of a chemical reaction depends on the rate of collisions between reactant particles. As reactant concentration is increased, the number of collisions per second is also increased, leading to an increase in the rate of the reaction.
C (s) + CO2 (g) <--> 2CO (g) H > 0
A sealed, rigid container holds an equilibrium mixture of the system represented above.
What change will increase the amount of CO (g) in the container?
Increasing the temperature of the system
Explain: Because the synthesis of CO (g) is endothermic, increasing the temperature of the equilibrium system will shift the equilibrium position to the right, toward the formation of CO (g).
H2O (g) --> H2O (l)
List the signs of H and S for the process represented above at 100 C and 1 atm?
H and S are negative.
H is negative because the increased intermolecular attractions between water molecules. S is negative because the water molecules have a smaller volume, resulting in decreased entropy.
A 2.70 vitamin C tablet is found to contain 0.0109 mol of ascorbic acid (C6H8O6). The molar mass of C6H8O6 is 176.12 g/mol.
What is the mass percent of C6H8O6 in the tablet?
71.1%
Explain: First, calculate the mass of C6H8O6 in the tablet. Then, using molar mass of C6H8O6, convert from moles to grams. Next, use the calculated mass of C6H8O6 and the total mass of the tablet to calculate mass percent.
A mixture of 4.0 mol He, 2.5 mol N2, and 3.5 mol Ar exerts a total pressure of 1.2 atm.
WHat is the partial pressure of the He?
0.48 atm
Explain: The partial pressure of a gas in a mixture of gases can be calculated using the equation:
P1 = Ptotal x X1
X(g) + 2Y(g) --> Z(g) + 2Q(g)
The reaction above is found to be zero order with respect to X and second order with respect to Y.
What happens to the rate of the reaction when [X] is doubled and [Y] is halved?
It decreases by a factor of 4
Explain: First, write out the rate law for the reaction. Since [Y] is raised to the second power in the rate law, decreasing [Y] by a factor of 2 results in the reaction rate decreasing by a factor of 4.
C (s) + H2O (g) <--> CO (g) + H2 (g)
What is the expression for the reaction quotient, Qc, for the reaction represented above?
Qc = [CO][H2]/[H2O]
Explain: Carbon is exempt because it is a solid.
CH4(g) + 2H2S(g) --> CS2(g) + 4H2(g) Hrxn > 0
Under which temperature conditions is the reaction represented above thermodynamically favored?
High temperatures only
Since Hrxn and Srxn are both positive, Grxn will be negative only at higher temperatures.
In which of the following ionic compounds is the force of attraction between cation and anion the strongest?
A) RbI
B) CsI
C) MgO
D) Srs
C) MgO
Explain: The internuclear distance is smallest in MgO meaning that the force of attraction between cation and anion is strongest in MgO.
Which of the following scenarios is an example of a chemical process?
A) A solution of H2O is heated, resulting in the evolution of oxygen gas.
B) A sample of solid iodine is placed under vacuum, causing it to sublime.
C A piece of copper metal is rolled and drawn out into a thin wire.
A) A solution of H2O is heated, resulting in the evolution of oxygen gas.
Explain: When a solution of H2O is heated, a new substance with a different chemical composition is formed. This can result only by breaking and forming of chemical bonds, so this scenario is an example of a chemical process.
A sample of marshmallow is combusted directly below a can containing 100 g of water initially at 25.0 C. The reaction releases 2.1 kJ of heat energy, all of which is transferred to the water.
What is the final temperature of the water? (The specific heat of water is 4.2 J/(g x C).)
30.0 C
Explain: The temperature change of the water can be calculated using the equation q = mcDeltaT
The pH of a sample of black coffee is 5.08 at 25 C.
Calculate [OH-] in the sample.
1.2 x 10-9
Explain: To calculate [OH-], you first need to find the pOH from the given value of pH. Then, you can use the pOH to determine [OH-].
Which reaction represented below is the standard entropy change positive (Srxn > 0)?
A) N2(g) + 3H2(g) --> 2NH3(g)
B) 2BaO2(s) --> 2BaO(s) + O2(g)
B) 2BaO2(s) --> 2BaO(s) + O2(g)
Explain: In general, the entropy change for a reaction is positive when the number of moles of gaseous products is greater than the number of moles of gaseous reactants.
Which of the following best predicts the type of bond that forms between S and F?
A) Nonpolar Covalent
B) Polar Covalent
C) Ionic
D) Metallic
B) Polar Covalent
Explain: In general, covalent bonds form between nonmetals, ionic bonds form between metals and nonmetals, and metallic bonds form between metals. Sulfur and fluorine are both nonmetals. Additionally, according to periodic trends, fluorine is significantly more electronegative than sulfur.
Write an equation that best represents the dissolution of Li2SO4 in water?
Li2SO4 (s) --> 2Li+ (aq) + SO42- (aq)
Explain: Li2SO4 is an ionic solid, which means it will dissociate into its constituent ions when dissolved in water. According to its formula, Li2SO4 will dissolve to form two Li+ ions for every one SO42- ion.
A 20 g iron rod is heated to a temperature T1 and then dropped into 20 g of water at lower temperature T2 in a polystyrene cup.
What is true of the final temperature of the system when thermal equilibrium is reached? (The specific heat of iron is 0.45 J/(g x C), and the specific heat of water is 4.2 J/(g x C).)
The final temperature is closer to T2 than to T1.
Explain: Since water has a greater specific heat than iron, the temperature change in the water will be smaller than the temperature change in the iron. As a result, the final temperature of the system will be closer to the initial temperature of the water than to the initial temperature of the iron.
Which of the following pairs of substances, when mixed in equimolar amounts, results in an acidic solution?
A) HBr (aq) and NH3 (aq)
B) HBr (aq) and NaOH (aq)
C) NaOH (aq) and NH3 (aq)
HBr (aq) and NH3 (aq)
Explain: Mixing equimolar amounts of a strong acid and a weak base produces a salt containing the conjugate acid of the weak base. The conjugate acid then reacts with water to form H3O+ ions, resulting in a weakly acidic solution.
Which reaction represented below is the standard entropy change positive (Srxn > 0)?
A) 2NO2(g) --> N2O4(g)
B) H2(g) + I2(s) --> 2HI(g)
B) H2(g) + I2(s) --> 2HI(g)
In general, the entropy change for a reaction is positive when the number of moles of gaseous products is greater than the number of moles of gaseous reactants.