Q: A photon of light has a wavelength of 400 nm. What is its energy in joules? ($$h = 6.626 \times 10$$^{-34}$$ J·s, c = 3.00$$ \times $$10^8 m/s)$$

$$E = \frac{hc}{\lambda} = \frac{6.626 \times 10$$^{-34}$$ \times 3.00 \times 10^8$}{400 \times 10$$^{-9}$$} = 4.97 \times 10$$^{-19}$$ J

Question 1

Which of the following compounds will produce the brightest light when its aqueous solution is tested with a conductivity apparatus, and why?

Accepted Answer

KNO$$_3$$, because it is a soluble ionic compound and completely dissociates.

Question 2

A student needs to prepare 250.0 mL of a 0.200 M NaCl solution from a 1.00 M NaCl stock solution. What volume of the stock solution should the student use?

Accepted Answer

$$M_1V_1 = M_2V_2$$; $$V_1$$ = \frac{M_2V_2}{$$M_1$$} = \frac{0.200 	imes 250.0}{1.00} = 50.0$$ mL

Question 3

Which of the following is the correct dissociation equation for Ca(NO$$_3$$)$$_2 in $$water, and what is the concentration of nitrate ions if the solution is 0.10 M?

Accepted Answer

Ca(NO$$_3$$)$$_2(aq$$) $$\rightarrow Ca$^{2+}(aq) + 2NO$_3^-(aq$$); [NO$$_3^-$$] = 0.20 M

Question 4

Which of the following statements about the first law of thermodynamics is correct?

Accepted Answer

$$\Delta E = q + w$$; energy is conserved and can only be transferred or converted.

Question 5

A reaction is carried out in a coffee-cup calorimeter. The temperature of the solution increases. Which of the following is true?

Accepted Answer

The reaction is exothermic and $$\Delta H \u003c 0.$$

Question 6

Calculate the amount of heat (in kJ) required to raise the temperature of 500.0 g of water from 20.0$$^\circC to 80.0$^\circC. ($$Specific heat of water = 4.184 J/g$$^\circC)$$

Accepted Answer

$$q = mc\Delta T = (500.0)(4.184)(60.0) = 125,520 J$$ $$= 125.5 kJ$$

Question 7

Given the reaction: $$2	ext{H}_2(g) + 	ext{O}_2(g) ightarrow 2	ext{H}_2	ext{O}(l)$$, $$\Delta H = -571.6 kJ. $$How much heat is released when 8.00 g of H$$_2$$ reacts completely?

Accepted Answer

Moles H$$_2$$: $$8.00 \div 2.016 = 3.97$$ mol; $$\Delta H$$ for 2 mol H$$_2 is$$ $$-571.6 kJ$$; $$3.97 	imes \frac{-571.6}{2} = -1,134 kJ$$

Question 8

Which of the following best describes the difference between a strong acid and a weak acid in aqueous solution?

Accepted Answer

A strong acid completely ionizes, while a weak acid only partially ionizes.

Question 9

Which of the following electron configurations is correct for a ground-state atom of chlorine (Cl, atomic number 17)?

Accepted Answer

1s$$^2 2s$^2 2p$^6 3s$^2 3p$^5$$

Question 10

A photon of light has a wavelength of 400 nm. What is its energy in joules? ($$h = 6.626 	imes 10$$^{-34}$$ J·s, c = 3.00$$ 	imes $$10^8 m/s)$$

Accepted Answer

$$E = \frac{hc}{\lambda} = \frac{6.626 	imes 10$$^{-34}$$ 	imes 3.00 	imes 10^8$}{400 	imes 10$$^{-9}$$} = 4.97 	imes 10$$^{-19}$$ J

Question 11

Which set of quantum numbers is not allowed for an electron in an atom?

Accepted Answer

$$n=2,\ l=2,\ m_l=0,\ m_s=+\frac{1}{2}$$

Question 12

Which of the following best explains why the 4s orbital fills before the 3d orbital in the ground-state electron configuration of potassium (K)?

Accepted Answer

The 4s orbital is lower in energy than the 3d orbital.

Question 13

Which of the following is a correct application of Hund's rule when filling the 2p orbitals for carbon (atomic number 6)?

Accepted Answer

Place one electron in each of two 2p orbitals with parallel spins before pairing.

Question 14

Which of the following elements is a strong electrolyte when dissolved in water, and why?

Accepted Answer

KBr, because it is a soluble ionic compound and completely dissociates.

Question 15

Which of the following best describes the energy change when an electron transitions from $$n=4 to$$ $$n=2 in a $$hydrogen atom?

Accepted Answer

Energy is released and a photon is emitted; $$\Delta E is $$negative.

Question 16

Which of the following ions will have the highest concentration in a 0.20 M solution of Na$$_2SO$_4$$?

Accepted Answer

[Na$$^+$$] = 2 	imes 0.20 = 0.40$ M

Question 17

Which of the following is the correct formula for calculating the enthalpy change of a reaction using standard enthalpies of formation?

Accepted Answer

$$\Delta H^\circ_{rxn} = \sum n\Delta H^\circ_f(	ext{products}) - \sum m\Delta H^\circ_f(	ext{reactants})$$

Question 18

Which of the following best explains why bright red light (long wavelength, low frequency) may not cause the photoelectric effect in a metal, even if the light is very intense?

Accepted Answer

Each photon does not have enough energy to eject an electron, regardless of intensity.

General Chemistry Study Guide: Chapters 4–6 (Aqueous Reactions, Thermochemistry, Electronic Structure)

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