2.7 Practice Problems

Attempt these problems as if they were real exam questions in an exam environment.

Only look up information if you get severely stuck. Never look at the solution until you have exhausted all efforts to solve the problem. Having to look up information or the solution should be an indicator that the previous layers (1–5) in the Structured Learning Approach have not been mastered.

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1. Studies were conducted on an unknown substance, where it was discovered that the substance has two stable isotopes. Using the information provided below, identify the average atomic mass of the unknown substance.

   % Abundance	Isotopic mass (amu)
   45.67	125.99938
   54.33	127.98762

   1. 126.64
   2. 127.08
   3. 128.15
   4. 125.99
   5. 124.00

   Solution

   Answer: B

   Concept: Average atomic mass

   \[\begin{align*} m_{\mathrm{avg}} &= \left [ \left ( \dfrac{45.67\%}{100} \right ) \left ( \dfrac{125.99938~\mathrm{amu}}{} \right ) \right ] + \left [ \left ( \dfrac{54.33\%}{100} \right ) \left ( \dfrac{127.98762~\mathrm{amu}}{} \right ) \right ]\\ &= 127.08~\mathrm{amu} \end{align*}\]

   
   

2. Which of the following statements describes a violation of the Law of Conservation of Mass?

   1. 100 grams of a reactant decomposes to produce 40 grams of one product and 60 grams of another.
      
   2. Four molecules of water decompose to produce four molecules of hydrogen and two molecules of oxygen.
      
   3. The same number of each atom type is present in the products of a chemical reaction as was present in the initial reactants.
      
   4. A 25 g sample of Reactant A combines with a 75 g sample of Reactant B (and nothing else), undergoes a complete chemical reaction to form 125 g of Substance C.

   Solution

   Answer: D

   Concept: Law of Conservation of Mass

   
   

3. Given the following table, which element sample would have the smallest number of atoms present?

   Element	Average Atomic Mass (amu)	Amount Present (g)
   Copper	63.55	78.00
   Zinc	65.38	99.60
   Carbon	12.00	5.6
   Hydrogen	1.01	15.50
   Sodium	22.99	40.2

   1. Copper
   2. Hydrogen
   3. Zinc
   4. Carbon
   5. Sodium

   Solution

   Answer: B

   Concept: Mass-to-mole conversion

   Average atomic masses in amu are numerically equivalent to average atomic masses in g mol^–1.

   \[\begin{align*} n_{\mathrm{Cu}} &= \left ( \dfrac{78.00~\mathrm{g}}{} \right ) \left ( \dfrac{1~\mathrm{mol}}{63.55~\mathrm{g}} \right ) = 1.227~\mathrm{mol~Cu} \\ n_{\mathrm{Zn}} &= \left ( \dfrac{99.60~\mathrm{g}}{} \right ) \left ( \dfrac{1~\mathrm{mol}}{65.38~\mathrm{g}} \right ) = 1.523~\mathrm{mol~Zn} \\ n_{\mathrm{C}} &= \left ( \dfrac{5.6~\mathrm{g}}{} \right ) \left ( \dfrac{1~\mathrm{mol}}{12.00~\mathrm{g}} \right ) = 0.47~\mathrm{mol~C} \\ n_{\mathrm{H}} &= \left ( \dfrac{15.50~\mathrm{g}}{} \right ) \left ( \dfrac{1~\mathrm{mol}}{1.01~\mathrm{g}} \right ) = 15.3~\mathrm{mol~H} \\ n_{\mathrm{Na}} &= \left ( \dfrac{40.2~\mathrm{g}}{} \right ) \left ( \dfrac{1~\mathrm{mol}}{22.99~\mathrm{g}} \right ) = 1.75~\mathrm{mol~Na} \\ \end{align*}\]

   
   

4. A compound has a molar mass of 450.45 g mol^–1 and an empirical formula of CH₂O. What is the molecular formula of the substance?

   1. CH₂O
   2. C₆H₁₂O₆
   3. C₁₁H₂₂O₁₁
   4. CHO
   5. C₁₅H₃₀O₁₅

   Solution

   Answer: E

   Concept: Molecular formula from empirical formula

   The molar mass of the empirical formula, CH₂O is 30.031 g mol^–1. Find the ratio of the molecular formula molar mass to the empirical formula molar mass.

   \[\begin{align*} \left( \dfrac{450.45~\mathrm{g}}{\mathrm{mol}}\right) \left( \dfrac{\mathrm{mol}}{30.031~\mathrm{g}}\right) = 14.999 \approx 15 \end{align*}\]

   Multiply the atoms in the empirical formula by the ratio determined above.

   \[\mathrm{C}_{1 \times 15} \mathrm{H}_{2 \times 15} \mathrm{O}_{1 \times 15} \longrightarrow \mathrm{C_{15}H_{\mathrm{30}}O_{15}}\]

   
   

5. How many carbon atoms are in 10.673 g of C₆H₁₂O₆?

   1. 4.24 × 10²⁴
   2. 6.02 × 10²³
   3. 5.08 × 10²³
   4. 2.14 × 10²³
   5. 9.90 × 10²⁴

   Solution

   Answer: B

   Concept: Mass to number of particles conversion

   \[\begin{align*} \mathrm{C~atoms} &= \left ( \dfrac{10.673~\mathrm{g~C_6H_{12}O_6}}{} \right ) \left ( \dfrac{\mathrm{mol~C_6H_{12}O_6}}{180.16~\mathrm{g}} \right ) \left ( \dfrac{6~\mathrm{mol~C~atoms}}{\mathrm{mol~C_6H_{12}O_6}}\right ) \left ( \dfrac{6.022\times 10^{23}~\mathrm{C~atoms}}{\mathrm{mol~C}} \right ) \\ &= 2.14\times 10^{23} \end{align*}\]

   
   

6. Which of the following descriptions is correct?

   1. In a neutral atom, the number of electrons is equal to the number of protons.
   2. The proton is a negatively charged subatomic particle found in the nucleus of atoms.
      
   3. The neutron is a charged subatomic particle found in the nucleus of some atoms.
      
   4. The electron is a small, negatively charged subatomic particle found in the nucleus of atoms.
      
   5. In an ion, the number of electrons is equal to the number of neutrons.

   Solution

   Answer: A

   Concept: Atomic structure

   
   

7. A sample contains only a single element. You run a mass spectrum to determine the mass of the individual atoms. You see that there are two peaks, separated by approximately 2 amu. Which of the conclusions below is the best?

   1. The element is composed of two isotopes, with the heavier isotope having 2 additional protons compared to the lighter isotope.
      
   2. The element is composed of two isotopes, with the heavier isotope having 2 additional neutrons compared to the lighter isotope.
   3. The element is composed of two different elements.
      
   4. The element is composed of two isotopes, with the heavier isotope having two fewer neutrons than the lighter isotope.
      
   5. The element is composed of two isotopes, with the heavier isotope having two fewer protons than the lighter isotope.

   Solution

   Answer: B

   Concept: Isotopes

   
   

8. What symbol would represent an ion with 18 electrons, 16 protons, and 16 neutrons?

   1. Cl
   2. S
   3. Ar
   4. S^2–
   5. S^–

   Solution

   Answer: D

   Concept: Atomic symbol

   
   

9. What is the mass (in g) of 2.45×10²¹ N atoms?

   1. 8.78
   2. 17.5
   3. 2.90×10^–4
   4. 0.0570
   5. 3,440

   Solution

   Answer: D

   Concept: Atoms to mass conversion

   \[\begin{align*} m &= \left ( \dfrac{2.45\times 10^{21}~\mathrm{atoms}}{} \right ) \left ( \dfrac{\mathrm{mol}}{6.022\times 10^{23}~\mathrm{atoms}} \right ) \left ( \dfrac{14.01~\mathrm{g}}{\mathrm{mol}} \right )\\[2ex] &= 0.0570~\mathrm{g} \end{align*}\]

   
   

10. What is the best nuclide symbol for zinc-68?

    1. ³⁰Zn
    2. ³⁰Zn-68
    3. ³⁸Zn
    4. ³⁰Zn-38
    5. ⁶⁸Zn

    Solution

    Answer: E

    Concept: Isotopes

    
    

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"Chemistry I: Online Resource" was prepared by Eric Van Dornshuld.    |    Release: Alpha 1.0
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