General Review: Answers to Review Problems
Topic A: Atoms, Molecules and Moles
1) Isotopes are atoms with the same number of protons (so they are the same element) but with
different numbers of neutrons. This different number of neutrons results in identical elements
having different mass numbers.
2) Barium is element number 56. This means that barium has 56 protons. In a neutral atom, the
number of electrons is equal to the number of protons, so this species has 56 electrons. The mass
number of any element is the sum of the number of protons and the number of neutrons.
Therefore, the number of neutrons in this isotope is 135 – 56 = 79.
3) What does a charge of +1 tell you about the number of electrons? What is the relationship
between the number of electrons and the number of protons? The element is Na.
4) If 6Ga is 60.4%, then what is the percentage of the isotope Ga ? Use the average atomic mass
(from the Periodic Table) to determine the atomic mass of xGa, using the equation to determine a
Atomic Mass of Ga = 70.93, therefore the mass number of Ga is 71.
5) The molar mass of a compound tells us the mass of one mole of the compound. What are the
units of molar mass? Can you manipulate these two pieces of data to give you those units?
Molar mass = 114.97 gmol
6) a) 1 mole of C 2H 6O molecules contains 2 moles of C atoms
Therefore: 3.20 moles of C H 6O molecules contains 2 x 3.20 = 6.40 moles of C atoms
b) Each mole of C 2H 6O molecules contains 6.02 x 10 molecules
Therefore: 3.20 moles of C 2H O molecules contains 3.20 x 6.02 x 10 = 1.93 x 10 molecules
c) From a) we know that there are 6.40 moles of C atoms in 3.20 moles of C H 6O molecules
Therefore, 6.40 moles of C atoms contains 6.40 x 6.02 x 10 = 3.85 x 10 atoms
General Review - 1 Each C 2H O molecule contains a total of 9 atoms.
Therefore, 3.20 moles of C2H 6O molecules contain 3.20 x 9 = 28.8 moles of atoms
The total number of atoms is 28.8 moles x 6.02 x 1023atomsmol ―1= 1.73 x 10 atoms
d) One molecule of C 2H 6O contains a total of 9 atoms
Therefore: x molecules of C 2H6O contains a total of 100 atoms
x = 11.1 molecules (in theory only, in practice you can’t have 11.1 molecules!)
e) Molar Mass of C 2H O = 46.068 gmol ―1
Mass Percent of C = 2 x 12.01 x 100 = 52.1% Mass Percent of O = 16.00 x 100 = 34.7%
f) One mole of C H 6O molecules contains 2 moles of C atoms, 6 moles of H atoms and 1 mole
of O atoms
Mole fraction of C = = 0.222
Mole fraction of H = = 0.667
Mole fraction of O = = 0.111
7) Convert moles of each species to mass. Express mass of SiO 2as a percentage of the total mass of
Mass % of SiO 2 = 40.3%
Mole % of SiO 2 = 39.6% Mole % of NaCl = 60.4%
8) Use a 100 g sample, therefore: C = 44.77 g, H = 7.52 g, O = 47.71 g
Convert to moles and determine the smallest whole number ratio of moles.
Actual ratio of moles is 1.25 C : 2.50 H : 1 O
Multiply by 4 to get whole numbers: 5 C : 10 H : 4 O
Formula is C 5H10O 4
9) For empirical formula, same approach as 8).
Ratio is 2 C : 5 H : 1 O, so empirical formula is H 5O (Molar Mass = 45.06 gmol )
Molecular formula is a multiple of simplest formula; determined using molar mass.
Molecular formula is C 4H 1O 2.
General Review - 2 10) Same approach as in 8) and 9)
Ratio is 2 C : 5.5 H : 1 N, so multiply by 2 to get whole numbers; 4 C : 11 H : 2 N
Empirical formula is C 4H 1N 2 (Molar Mass = 87.148 gmol ) ―1
As molar mass of empirical formula is the same as the molar mass of molecular formula;
the molecular formula is C 4H 1N 2.
11) We are using 1.00 x 10 23molecules of oxygen; ie, O ! Find moles of O 2needed. (Use Avogadro’s
How many moles of O 2are needed to form 1 mole of MgSO 4?
Mass of MgSO 4= 0.0830 moles x 120.38 gmol ―1 = 9.99 g
12) Use a variable (x) to determine the molar mass of C H 12O x;
Molar mass = 5(12.01) + 12(1.008) + x(16.00) = 72.146 + 16.00x
Use mass percent of H to determine x.
∴ x = 3
13) What is the relationship between moles of OsO 4 and moles of Os in OsO ?
Atoms Os = 2.37 x 10 atoms
14) Start with 100 g of Ne and 100 g of Ar. (You could use any mass here.)
Then convert to moles and find mole fraction.
X Ne = 0.664
15) What is the relationship between moles of C H 14and moles C in C 6H 1?
Carbon is 1.11% 1C, ∴ moles C = moles Carbon x 0.0111 = 0.333 moles
16) What does % by mass O actually mean? Write the relationship and use a variable for the molar
mass of M in that relationship. Solve for M.
M = 56 gmol ; the element is Fe
17) a) How many moles of C 8H NO 2 are in a 500 mg tablet?
Number of molecules = 1.99 x 10 21
b) One trillion = 1 x 1012
Number of molecules accepted by each cell = 1.99 x 10 8molecules
General Review - 3 18) What is the Average Atomic Mass of Boron (from Periodic Table)?
Use this in a weighted average.
Let Fraction B = x, ∴ Fraction B = 1 - x
Percent Abundance of B = 20.0 %
19) What is the % by mass of Fe in Fe 2O ?
Any sample of Fe O 3will be that % of Fe.
A 65.2 g sample of Fe 2O 3will contain 45.6 g Fe
Topic B: Solutions
1) What is the definit