The total gram-molecular weight of this substance, then, is
100 g. Oxygen’s contribution is 64 g, which means the
compound is 64 percent oxygen by mass.
- E As soon as you hear the term electron sharing, you know you’re dealing
with a covalent bond, so eliminate (A), (B), and (C). The fact that the two
atoms differ in electronegativity tells you that one has more attraction for
the shared electrons than the other. The result? A polar covalent bond—
the molecule has a negative and a positive pole.
- B Use q = mc∆T to compute the amount of heat transfer. For water, the
specific heat, c, is about 1 calorie/g • °C, so a 20 g sample of water
experiencing a 20°C increase in temperature has (20 g)(1 calorie/g • °C)
(20° C), or 400 calories of heat, transferred to it.
- D Remember the oxidation state rules. An oxygen atom usually has a −2
state. Potassium atoms are always given a +1 state. In K 2 Cr 2 O 7 , we have
2 potassium atoms and 7 oxygen atoms. So potassium atoms contribute
2(+1), or a state of +2. Oxygen atoms contribute 2(−7), or −14. For
K 2 Cr 2 O 7 to be neutral, each chromium atom must have a state of +6.
- B If the pH is 5, then [H+] is 1 × 10−5 moles/L. Water’s ion product is 1 ×
10 −14 at 25°C, meaning that the product [H+] × [OH−] is 1 × 10−14. So
[OH−] = (1 × 10−14)/(1 × 10−5) = 1 × 10−9 moles/L. Choice (B) is correct.
- D Based on the balanced equation, the ratio of water vapor consumed to
oxygen produced is 2 moles H 2 O to 1 mole O 2 . The volume of gas will
also be in this 2:1 ratio. So 89.6 liters of H 2 O(g) are required to produce
44.8 liters of O 2 (g).
- B Molarity refers to moles of solute per liter of solution. We know we have
500 milliliters of solution, but we don’t know how many moles of solute
we have. Let’s first figure out the mass of 1 mole of MgCl 2 . Looking at
the periodic table, we find that 1 mole of Mg has a mass of 24.3 g. Two
moles of Cl have a mass of about 71 g. One mole of MgCl 2 , therefore,
has a mass of 95.3 g.
We can then determine the moles of MgCl 2 by dividing the
