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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

(C) 2.0 g, 3.0 g, 4.0 g (D) 1 g, 3 g, 5 g (E) 2.0 g, 3.0 g, 4.0 g, 5.0 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 g

 

(c) liters to centiliters?

 

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

 

(c) binary molecular compound? (A) BeHCO3 (B) PCl5 (C)AgI (D)MgS (E)Cr2O3

 

(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 MxTy. 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 C (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) C7H14 and 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

 

C2F4

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

2

 

 

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) 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) 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.

 

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.

 

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.