Answers to the Chemistry Final Exam Review Questions

Honors Chemistry

Midterm Exam Review

ANSWER KEY

1. What are the differences between a mixture and a compound?

Mixtures can be separated into their components by physical means. Compounds can be separated into their components only by chemical means.

2. What must occur for a process to be a chemical reaction?

The process must involve a change in chemical properties.

3. Which group of measurements is the most precise? (Each group of measurements is for a different object. (A) 2 g, 3 g, 4 g (B) 2 g, 2.5 g, 3 g

(B) 2g, 2.5g, 3g

4. When multiplying and dividing measured quantities, the number of significant figures in the result should be equal to the number of significant figures in _____.

The least precise measurement

5. What a correct conversion factor for changing:

(a) kilometers to meters?

1000 m = 1 km

(b) grams to micrograms?

1 g = 1 x 106 µg

1 L = 100 cL

6. The density of osmium, which is the densest metal, is 22.57 g/cm³. What is the mass of a block of osmium that measures 1.00 cm x 4.00 cm x 2.50 cm?

M M = DV

D = ------ = (22.57 g/cm3)(1cm x 4 cm x 2.5 cm)

V = 226 g

7. What are the components of Dalton's atomic theory?

All elements are composed of tiny indivisible particles called atoms. Atoms of the same element are identical. Atoms of different elements are different. Atoms of different elements can physically mix together or change chemically by combining in simple whole number ratios to form compounds. Chemical reactions occur when atoms are separated, joined or rearranged. Atoms of one element never change into atoms of another element by chemical reactions.

8. How do the isotopes hydrogen-1 and hydrogen-2 differ?

Hydrogen-1 has no neutrons.

Hydrogen-2 has one neutron.

9. Why do chemists use relative comparisons of masses of atoms?

The actual mass of protons and neutrons is very small.

10. Isotopes of the same element have different _____.

mass number

11. If E is the symbol for an element, which two of the following symbols represent isotopes of the same element?

(A) 20 (B) 20 (C) 21 (D) 21

E E E E

10 11 9 10

(A) 20 (D) 21

E and E

10 10

12. Consider an element Z that has two naturally occurring isotopes with the following percent abundances: the isotope with a mass number of 20 is 25% abundant; the isotope with a mass number of 22 is 75% abundant. What is the average atomic mass for element Z?

20 x 0.25 = 5 g

22 x 0.75 = 16.5 g

--------

21.5 g

13. Which subatomic particle plays the greatest part in determining the Physical and chemical properties of an element?

electron

14. The modern periodic table is arranged in order of increasing atomic ____.

number

15. In any chemical compound, the elements are always combined in the same proportion by ___.

mass

16. Which of the following formulas represents:

(a) an ionic compound?(A)BaI₂ (B) N₂O₄(C)Kr (D)CS₂ (E) PCl₃

(A) BaI₂

(b) a molecular compound? (A) Xe (B)SO₂ (C) BeF₂ (D)ZnO (E) Mg₃N₂

(B)SO₂

(B) PCl₅

(d) When naming acids, the prefix hydro- is used when the name of the acid anion ends in _____.

“-ide”

17. (a) When Group 2 elements form ions, they ___ two electrons.

lose

(b) The nonmetals in Group 17 _____ one electron when they form ions.

gain

18. (a) An -ite or -ate ending on the name of a compound indicates that the compound _____.

contains a polyatomic ion

(b) Ternary ionic compounds contain three different _____.

elements

19. Which of the following compounds contains the lead(IV) ion? (A) PbO₂ (B) PbCl₂ (C) Pb₄O₃ (D) PbO (E)Pb₂O

(A) PbO₂

20. Consider a mystery compound having the formula M_xT_y. If the compound is not an acid, if it contains only two elements and if M is not a metal; which of the following is true about the compound? (A) It contains a polyatomic ion. (B) It is a binary molecular compound. (C) Its name ends in -ic. (D) Its name ends in -ite or -ate. (E) It is a binary ionic compound.

(B) It is a binary molecular compound.

21. The lowest whole-number ratio of the elements in a compound is called the _____.

empirical formula

22. (a) The molar volume of a gas at STP occupies _____.

22.4 L

(b) What combination of temperature and pressure correctly describes standard temperature and pressure, STP?

0 ºC (273 K) and 1 atm (101.3 kPa)

23. Which of the following compounds has the lowest percent gold content by weight?(A) AuCl₃ (B) AuBr₃ (C) AuI₃ (D) Au(OH)₃ (E) AuOH

(C) AuI₃

24. Which of the following compounds have the same empirical formula? (A) CO₂ and SO₂(B) NO and NO₂(C) C₇H₁₄and C₁₀H₂(D) C₄H₁₀ and C₁₀H₄(E) C₆H₁₂ and C₆H₁₄

(C) C₇H₁₄ and C₁₀H₂₀

25. What is the percent composition of NiO, if a sample of NiO with a mass of 10.3 g contains 8.1 g Ni and 2.2 g O?

8.1 g

%Ni = -------- X 100% = 79 % Ni

10.3 g

2.2 g

%O = -------- X 100% = 21 % O

10.3 g

26. The percentage composition of a polymer used for the non- stick surfaces of cooking utensils, is 24.0% C and 76.0% F by mass. What is the empirical formula of the polymer?

24.0 g C 76.0 g F

----------- = 2 mol C ----------- = 4 mol F

12.0 g C 19.0 g F

C₂F₄

-------- = CF₂ (to get the simplest whole number ratio)

27. The ratio of carbon to hydrogen to nitrogen atoms in nicotine is 5 to 7 to 1. What is the molecular formula of nicotine if its gram molecular mass is 162 g? The empirical formula is C₅H₇N.

Empirical Formula Mass

C: 12.0 x 5 = 60.0 g 162 g

H: 1.0 x 7 = 7.0 g -------- = 2

N: 14.0 x 1 = 14.0 g 81.0 g

---------

81.0 g

(C₅H₇N) x 2 = C₁₀H₁₄N₂

28. If 4.0 g of H₂ are made to react with excess CO, how many grams of CH₃OH can theoretically be produced according to the following equation? CO(g) + 2H₂ (g) à CH₃OH(l)

4.0 gH₂ | 1 mol H₂ | 1 mol CH₃OH | 32 gCH₃OH

----------|-------------|-------------------|------------------ = 32g CH₃OH

1 | 2 gH₂ | 2 mol H₂ | 1 mol CH₃OH

29. How many grams of butane (C₄ H₁₀) must be burned in an excess of O₂ to produce 15.0 g of CO₂?

2C₄ H₁₀ (g) + 13O₂ (g) à 8CO₂ (g) + 10H₂O(g)

15.0 gCO₂ | 1 mol CO₂ | 2 mol C₄H₁₀ | 58.0 g C₄H₁₀

-------------|---------------|-----------------|---------------- = 4.94 g C₄H₁₀

1 | 44 g CO₂ | 8 mol CO₂ | 1 mol C₄H₁₀

30. What is the importance of the coefficients in a balanced chemical reaction? (9)

The coefficients in a balanced chemical equation indicate the relative number of moles of reactants and products. From this information the amounts of reactants and products can be calculated. The number of moles may be converted to mass, volume, or number representative particles.

31. In which kind of stoichiometric calculation can the steps involving conversion to and from moles be omitted? Explain why it is possible to do so. (9)

Volume-volume conversions between gases do not require mole conversions. Molar volumes of all gases at STP are the same. The coefficients in a balanced equation indicate the relative number of moles and the relative volumes of interacting gases.

32. Explain why it takes more energy to remove a 4s electron from zinc than from calcium.

Electrons occupy orbitals in a definite sequence, filling orbitals with lower energies first. Generally, orbitals in a lower energy level have lower energies that those in a higher energy level, but in the third level the energy ranges of the principal energy levels begin to overlap. As a result, the 4s sublevel is lower in energy than the 3d sublevel, so it fills first.

33. Give the electron configuration for a neutral atom of chlorine. (13).

1s²2s²2p⁶3s²3p⁵

34. Describe the different principles that govern the building of an electron configuration.

The Aufbau principle states that electrons enter the orbitals of lowest energy first. The Pauli exclusion principle states that each orbital can hold only two electrons. Hund's rule states that electrons first enter separate orbitals of the same energy, with each electron having the same spin, before sharing an orbital or having different spins.

35. Explain why the 4s sublevel fills before the 3d sublevel begins to fill as electrons are added.

36. What is the quantum mechanical model?

It is a model that describes the motions of electrons in atoms as probabilistic motions within a certain region. It is depicted as electron clouds, the density of which represents the probability of finding the electron in that region. The electron cloud of the quantum mechanical model is centered on the atomic orbital as proposed by Bohr.