The reaction is an example of β− decay, so X is an electron ( ) emitted by this type of
radioactive decay.59 . B
See the explanation to question 58.
60 . C
In the equation 2H 2 + O 2 → 2H 2 O, if you begin with 4 g of H 2 at 2 g/mole, you have 2 mol of H 2.
According to the equation, a 100% yield would result in 2 moles of H 2 O. Two moles of H 2 O at 18
g/mol would weigh 36 grams. Percent yield is defined as actual/theoretical × 100%. (27/36) ×
100% = 75%.
61 . A
An increase in entropy is an increase in randomness and loss of order. Gas going to a solid is an
increase in order and two molecules combining to form one molecule is also an increase in
order. A solid (Na) becoming an ionic compound (NaOH) and a gas (H 2 ) is a decrease in order
and an increase in entropy.
62 . C
This question requires you to know how to calculate the percentage by weight of an element
in a compound. You need to first calculate the weight of oxygen in each compound and then
divide that value by the compound’s molecular weight. The weight of oxygen in choice A is 80
grams/mole and the MW = 157 grams/mole. This is 51% oxygen. Doing the same with the other
answer choices, you get 43% oxygen (choice B), 53% oxygen (choice C), and 46% oxygen
(choice D). Obviously, choice C contains the most oxygen by weight and is the correct answer.
63 . C
If you have 0.25 mole of SO 2 and 0.25 mole of O 2 , SO 2 is the limiting reagent since you need
two moles per reaction compared to one mole of O 2 . Since 2 moles of SO 2 yields 2 moles of
SO 3 , 0.25 mole of SO 2 would yield 0.25 mole of SO 3.
64 . A
A buffer solution is prepared from a weak acid and its conjugate base in near equal quantities.
As long as these conditions are met, the pH should remain the same. A buffer solution with the
concentrations of each of these components halved may have less ability to buffer, but the
initial solution will have the same pH.