88 g C 3 H 8 /44g/mol = 2 mol; therefore, 7 mol of oxygen are needed.7 mol O 2 × 32 g/mol = 224 g of O 246 . C
A net charge of +1 has one more proton than electrons. Therefore, only choices B or C may be
correct. Mass number of 75 means that the protons plus neutrons must equal 75. This
eliminates answer choice B.
47 . D
half-lives 0 1 2 3 4
grams remaining 320 160 80 40 20four half-lives in 20 minutes20 minutes divided by 4 = 5 minutes48 . D
When the periodic table was first being designed, it was thought that the periodicity of the
elements could be explained on the basis of atomic mass. Mendeleev discovered that when
the elements were arranged in order of increasing atomic mass, certain chemical properties
were repeated at regular intervals. However, certain elements could not be fit into any group
of a table based on increasing atomic mass. It was the discovery of the nucleus and its
components that led scientists to order the elements by increasing atomic number, the
number of protons.
49 . D
A catalyst is a substance that in relatively small amounts will accelerate both the forward and
reverse reactions by lowering the activation energy, without being changed itself. It can be as
simple as a proton (acid catalysis) or a big biomolecule (in living organisms).
50 . B
An alpha particle is defined as and a beta particle is defined as (an electron).