Q: A reaction has a theoretical yield of 10.0 g, but only 7.5 g of product is recovered. What is the percent yield?

$$\displaystyle \frac{7.5\ \mathrm{g}}{10.0\ \mathrm{g}} \times 100\% = 75\%$$

Question 1

Which of the following best explains why $$\mathrm{CO_2$} is a $$greenhouse gas, but $$\mathrm{N_2$}$$ and $$\mathrm{O_2$}$$ are not?

Accepted Answer

Because $$\mathrm{CO_2$}$$ has vibrational modes that cause symmetry breaking, allowing it to absorb infrared radiation.

Question 2

A student is given the unbalanced equation for the combustion of propane: $$\mathrm{C_3H_8 + O_2$ \rightarrow CO_2$ + H_2O}. $$What is the correct balanced equation?

Accepted Answer

$$\mathrm{C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O}$$

Question 3

Which of the following statements about Beer's Law is correct?

Accepted Answer

According to Beer's Law, absorbance ($$A) $$increases linearly with both concentration ($$c) $$and path length ($$b$$): $$A = \varepsilon b c$$

Question 4

A reaction has a theoretical yield of 10.0 g, but only 7.5 g of product is recovered. What is the percent yield?

Accepted Answer

$$\displaystyle \frac{7.5\ \mathrm{g}}{10.0\ \mathrm{g}} 	imes 100\% = 75\%$$

Question 5

Which of the following best describes a limiting reactant in a chemical reaction?

Accepted Answer

It is the reactant that is completely consumed first, thus limiting the amount of product formed.

Question 6

Which of the following is a correct net ionic equation for the reaction between aqueous solutions of $$\mathrm{Na_2SO_4}$$ and $$\mathrm{$$BaCl_2$$}$$?

Accepted Answer

$$\mathrm{Ba$$^{2+}$$ (aq) + SO_4$^{2-} (aq) \rightarrow $$BaSO_4$$ (s)}$$

Question 7

A 0.250 L solution contains 0.0500 mol of $$\mathrm{NaCl}. $$What is the molarity of the solution?

Accepted Answer

$$\displaystyle M = \frac{0.0500\ \mathrm{mol}}{0.250\ \mathrm{L}} = 0.200\ \mathrm{M}$$

Question 8

Which of the following best describes a strong electrolyte?

Accepted Answer

A substance that completely dissociates into ions when dissolved in water.

Question 9

Which of the following is the correct Arrhenius definition of a base?

Accepted Answer

A substance that increases the concentration of $$\mathrm{OH^-}$$ ions in aqueous solution.

Question 10

Which of the following is a correct example of a formation reaction?

Accepted Answer

$$\mathrm{Na (s) + \frac{1}{2} Cl_2$ (g) \rightarrow NaCl (s)}$$

Question 11

A sample of gas does 150 J of work on the surroundings while absorbing 200 J of heat. What is the change in internal energy ($$\Delta E) of $$the system?

Accepted Answer

$$\Delta E = q + w = 200\ \mathrm{J} + (-150\ \mathrm{J}) = 50\ \mathrm{J}$$

Question 12

Which of the following best describes the sign conventions for $$q$$ and $$w in $$the First Law of Thermodynamics?

Accepted Answer

$$q is $$positive when heat is absorbed by the system; $$w is $$positive when work is done on the system.

Question 13

Which of the following statements about water is correct and helps explain its high 'water footprint' in industrial and agricultural processes?

Accepted Answer

Water has a high specific heat capacity, allowing it to absorb and release large amounts of heat with little temperature change.

Question 14

Which of the following is the correct expression for Coulomb's Law describing the electrostatic potential energy between two ions?

Accepted Answer

$$E = \displaystyle \frac{k Q_1$ Q_2$}{r}$$

Question 15

Which of the following is a state function?

Accepted Answer

Enthalpy ($$H)$$

Question 16

Which of the following best describes a precipitation reaction?

Accepted Answer

A reaction in which two aqueous solutions combine to form an insoluble solid.

Question 17

In the reaction $$\mathrm{Zn (s) + Cu$$^{2+}$$ (aq) \rightarrow Zn$$^{2+}$$ (aq) + Cu (s)}$$, which species is oxidized and which is reduced?

Accepted Answer

Zn is oxidized; $$\mathrm{Cu$$^{2+}$$} is $$reduced.

Question 18

A 100.0 g sample of water is heated from 25.0 $$^\circC to 75.0$$ $$^\circC. If $$the specific heat of water is $$4.18\ \mathrm{J\ g$$^{-1}$$\ ^\circ C$$^{-1}$$}$$, how much heat is absorbed?

Accepted Answer

$$q = m c \Delta T = 100.0\ \mathrm{g} 	imes 4.18\ \mathrm{J\ g$$^{-1}$$\ ^\circ C$$^{-1}$$} 	imes (75.0 - 25.0)\ ^\circ\mathrm{C} = 20,900\ \mathrm{J}$$

Question 19

Which of the following best describes the role of a reducing agent in a redox reaction?

Accepted Answer

It donates electrons and is itself oxidized.

Chemistry 1770 Exam #3 Study Guide: Stoichiometry, Solution Chemistry, and Energy

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