Chapter 2 Atoms, Molecules and Ions

Example: Ammonia (NH₃) is a compound of nitrogen and hydrogen, and it is established that the mass ratios of ammonia to nitrogen and hydrogen are:

Based on the law of multiple proportions, how many grams of ammonia would you expect to obtain from 2.34 g of nitrogen to combine to yield ammonia? How many grams of hydrogen?

Since the ratio of ammonia to nitrogen is 17g_ammonia: 14g_nitrogen, you can do simple dimensional analysis: $$?g_{ammonia}=2.34g_{nitrogen}\left(\frac{17g_{ammonia}}{14g_{nitrogen}}\right)=2.84544g=2.8g$$ Similar calculation can be done with hydrogen, and yields 0.50 g.

Example: 2.72 A sample of mercury with a mass of 114.0 g was combined with 12.8 g of oxygen gas, and the resulting reaction gave 123.1 g of mercury(II) oxide. How much oxygen was left over after the reaction was complete?

From convervation of mass in chemical processes, all the reactants combined have the same mass as all the products. $$m_{react}=m_{prod}$$ $$m_{Hg}+m_{O_2}=m_{HgO}+m_{O_2}$$ $$114.0g+12.8g=123.1g+m_{O_2}$$ Solving for m_O2 = 3.70 g

Example: 2.76 Benzene, ethane, and ethylene are just three of a large number of hydrocarbons, compounds containing only carbon and hydrogen. Show how the following data are consistent with the law of multiple proportions.

The law of multiple proportion states that compounds are composed of integral multiple of simple proportions. For example, if we have a compound which has one carbon and one hydrogen, the mass ratio between C and H are 12:1. If one carbon and two hydrogen, then 12:2 or 6:1. If one carbon and three hydrogen, then 12:3 or 4:1. These are due to the fact that the molar mass of carbon is 12 and of hydrogen is 1. From these, benzene the ratio is 4.61:0.39 which reduces to be 11.8:1. Given the error in the experiment, we can say it is 12:1. It means that benzene has 12:1 mass ratio for C to H. Indeed benzene's chemical formula is C₆H₆ therefore 6(12):6(1) or 12:1.

Ethane has 4:1, and its chemical formula is C₂H₆. The simplest numerical ration is CH₃. Using the chemical formula C₂H₆ 2(12):6(1) = 24:6 = 4:1, which is the same as the CH₃.

Similar analysis can be done on ethylene, C:H = 4.29g:0.71g = 6:1, which is one carbon and two hydrogens. Ethylene's chemical formula is C₂H₄, therefore reduces to CH₂ as the simplest numerial ratio.

Example: 2.100 How many protons, neutrons, and electrons are in each of the following atoms?    a. ${}_7{}^{15}N$    b. ${}_{27}{}^{60}\mathrm{Co}$    c. ${}_{53}{}^{131}I$    d. ${}_{58}{}^{148}\mathrm{Ce}$

a. The subscript gives the number of protons, Z, the nuclear charge. An atom is electrically neutral, meaning that the numbers of protons and electrons are the same. The superscript is the mass number, A, which is given by A = Z + N where N is the number of neutrons. From these, the number of protons = 7, number of neutrons = 8, and the number of electrons = 7.

b. Using the arguments in a. we have Z = 27, N = 33, # e^- = 27

c. Z = 53, N = 78, # e^- = 53

d. Z = 58, N = 90, # e^- = 58

Example: 2.121 Which of the following bonds are likely to be covalent and/or ionic? Explain. a. B—Br     b. Na—Br     c. Br—Cl     d. O—Br

Determination of whether the chemical bond is covalent is nature or ionic in nature is simply to look at the periodic table. If the two atoms in question appear far apart on the periodic table, it is likely to have ionic bond. If they appear nearby, it is likely to be covalent.

a. From this assessment, B—Br is ionic.

b. Na—Br is ionic.

c. Br—Cl is covalent.

d. O—Br is covalent.

Example: 2.124 How many protons and electrons are in each of the following ions?    a. $B{e}^{2+}$    b. $R{b}^{1+}$    c. $S{e}^{2-}$    d. $A{u}^{3+}$

a. Atomic number of Be is 4. So, it means that Be atom has four electrons. In Be²⁺ ion, there are only 2 electrons. The superscript, 2+, indicates that the ion is missing 2 electrons.

b. Z = 37, # of e^- = 36

c. Z = 34, # of e^- = 36

d. Z = 79, # of e^- = 76

Example: 2.140 What are the formulas of the compounds formed from the following ions?

a. Ca²⁺ and Br^-     b. Ca²⁺ and SO₄^2-     c. Al³⁺ and SO₄^2-

When compounds are electrically neutral. So, when combine the two ions, you must make the compound comes out to be electrically neutral by thinking of integral multiples of ions.

a.Since Ca²⁺ has +2 charge, you need a -2 charge to cancel out. Since Br^- has -1 charge, you need two Br^- ions. Therefore, the chemical formula is CaBr₂.

b. CaSO₄

c. Al₂(SO₄)₃