Chem 1405 homework chap2
1. An analysis of a 5.0 g sample pyrite ("fool's gold") revealed that it contained 2.3 g of
iron and 2.7 g of sulfur. What is the mass percentage of iron in fool's gold? (Section 2.1)
a. 54% b. 46% c. 2.3%
Hint: The percent composition of a compound = the mass of the element/total mass.
Fe%=2.3g/5.0g=0.46= 46%
2. Calculate the atomic mass of strontium, which has four naturally occurring isotopes:
Natural abundance (%) Atomic mass ( amu)
0.56
83.91
9.86
85.91
7.02
86.91
82.56
87.91
a. 54.66 b. 87.62 c. 892.3
Hint: refer to topic of average atomic mass. [(83.91 amu x 0.56)+ (85.91 amu x 9.86)+
(86.91 amu x 7.02)+ (87.91 amu x 82.56)]/100=87.62 amu
3. A potassium atom contains 19 protons, 19 electrons, and 20 neutrons. What is the
charge on a potassium ion that contains 19 protons, 18 electrons, and 20 neutrons?
(Section 2.4)
a. 0 b. -1 c. +1
4. Complete the following table by filling in the blanks.
Protons
Neutrons
Electrons
Mass( amu) Element
19
20
19
39
K
34
45
34
79
Se
20
20
20
40
Ca
11
12
11
23
Na
Hint: Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
For a neutral atom, number of protons = number of electrons
5. Which of the following statements about the isotopes of an element is true? (Section
2.5)
a. Atoms that are isotopes of one another have different atomic masses.
b. Atoms that are isotopes of one another have different numbers of electrons.
c. Atoms that are isotopes of one another have different atomic numbers.
6. Which of the following pairs of elements is likely to undergo similar chemical
reactions? (Section 2.6)
a. Li and Be b. Cl and Br c. K and F
Hint: elements of the same group have similar chemical propertities.
7. How many valence electrons does carbon have? (Section 2.10)
a. 2 b. 4 c. 14
Hint: the electronic configuration of carbon is 1s22s22p2, there are four valence electrons;
the other way to solve the problem is that for main group elements the number of valence
electrons is the group number. Carbon is an element of group IV, so there are four
valence electrons.
8. Which elements are described by the following electron configurations?
( a) 1s22s22p63s2 ( b) 1s22s22p63s23p4 ( c) 1s22s22p63s23p64s1
a. Mg, S, K b. Al, S, Na c. Mg, O, K
Hint: For a neutral atom, the number of electrons equals to the number of protons, which
is the atomic number. To solve a problem like this is to add up all the electrons. For
example, ( a) 1s22s22p63s2, 2+2+6+2=12, so the element must Mg.
9. Which elements are described by the following electron configurations?
( a) [ He] 2s22p1; ( b) [ He] 2s22p5; ( c) [ Ar] 4s2
a. B, S, K
b. Al, F, Na c. B, F, Ca
Hint: The question is very similar to question8. For example ( c) [ Ar] 4s2, add up all the
electrons, 18 +2=20, the element is Ca.
10. A negatively charged anion M-2 has an electronic configuration of 1s22s22p6, what is
the element M?
a. F
b. Ca
c. O
Hint: An element gains electron to form negatively charged anions, M + n e- M-n. So
the anion must have n more electrons than the original element. In this case, the anion
has 2+2+6=10 electrons. Hence the original element has 10-2(the number of charge)=8
electrons. The element is O.