Quiz 2 answers - with worked solutions
1. What is average mass, in grams, of one atoms of iron?
use mol wt of iron (= molar mass = mass for one mol of Fe atoms) in g/mol and convert to g/atom using
Avagadro's number.
(55.85 g/mol) (1 mol / 6.022 x 1023 atoms) = 9.27 x 10-23 g/atom
2. What is the molar mass of acetaminophen, C8H9NO2?
molar mass is given in g/mol and the same as molecular weight which can be calculated from the sum of the
atomic weights for the atoms in the molecule:
8(12.01 g/mol) + 9(1.008 g.mol) + (14.01 g/mol) + 1(16.00 g/mol) = 151.16 g/mol
3. Formaldehyde has the formula CH2O. How many moles are there in 0.11 g of formaldehyde?
convert grams to moles by dividing mass by mol wt:
CH2O mol wt = 12.01 g/mol + 2(1.008 g/mol) + 16.00 g/mol = 30.03 g/mol
0.11 g / 30.03 g/mol = 3.7 x 10-3 moles
4. Which of the following samples contains the greatest number of atoms?
convert all values to same number unit (moles works) for direct comparison:
A. 100 g of Pb
B. 2.0 mole of Ar
C. 0.1 mole of Fe
D. 5 g of He
E. 20 million O2 molecules
100 g / 207.2 g/mol = 0.483 moles
2.0 moles
0.1 moles
5 g / 4.003 g/mol = 1.25 moles
40,000,000 atoms / 6.022 x 1023 atoms/mol = 6.64 x 10-17 moles
5. How many moles of oxygen atoms are there in 10 moles of KClO3?
there are 3 atoms of oxygen per molecule, so 10 moles of molecules would be 30 moles of oxygen atoms.
6. How many fluorine atoms are there in 65 g of CF4?
convert grams of CF4 to moles of CF4, multiply by Avagadro's number to convert to # of CF4 molecules,
multiple by 4 (because of 4 F atoms per molecule) to get # of F atoms.
mol wt (CF4) = 12.01 g/mol + 4(19.00 g/mol) = 88.01 g/mol
65 g / 88.01 g/mol = 0.74 mol CF4
0.74 mol x 6.022 x 1023 molecules/mol = 4.5 x 1023 molecules of CF4
(4.5 x 1023 molecules) x 4 F-atoms/molecule = 1.8 x 1024 F atoms
7. A compound was discovered whose composition by mass is 85.6% C and 14.4% H. Which of
these choices could be the molecular formula of this compound?
consider 100 g of the compound and determine relative number of moles of the elements present, then convert
the relative amounts of each element to the simplest whole-number ratio to give the empirical formula. The
molecular formula can be some multiple of the empirical formula.
85.6 g / 12.01 g/mol = 7.13 moles C
14.4 g / 1.008 g.mol = 14.3 moles H
C7.13H14.3 = CH2, so possible molecular formula is B. C2H4
8. Consider the reaction below.
MnO2 + 4 HCl -----> MnCl2 + Cl2 + 2 H2O
If 0.90 mole of MnO2 and 48.2 g of HCl react, which reagent will be used up first? (i.e. which is the
limiting reagent?)
compare mol MnO2 with mol HCl. the required rxn ratio is 1:4. the reactant present in excess of the 1:4 ratio
will not be used up in the reaction, so the other reactant will be used up first.
48.2 g / 36.46 g/mol = 0.132 mol HCl
0.9 mol MnO2
0.9 mol MnO2 requires 4(0.9)=3.6 mol HCl for complete rxn, but only 0.132 mol HCl is present, so HCl is the
limiting reagent and will be consumed first.
9. When balanced with the smallest set of whole numbers, the coefficient of O2 in the following
equation is:
__ C2H4 + __ O2 -----> __ CO2 + __ H2O
balance C first and then H and last O to give:
C2H4 + 3 O2 -----> 2 CO2 + 2 H2O
10. Propane (C3H8) is a component of natural gas and is used in domestic cooking and heating. The
balanced reaction for its combustion is given by which of the following.
A. C3H8(g) + 5 O2(g) ----> 3 CO2(g) + 4 H2O(l)
B. C3H8(g) + 5 O2(g) ----> 3 CO2(g) + 3 H2O(l)
C. C3H8(g) + 5 O2(g) ----> 2 CO3(g) + 4 H2O(l)
D. C3H8(g) + 4 O2(g) ----> 2 CO3(g) + 4 H2O(l)
11. According to the balanced reaction, how many grams of carbon dioxide is produced by burning
3.85 moles of propane? Assume that oxygen is the excess reagent in this reaction.
_______g (use 3 significant figures)
multiply mol propane by 3 (due to rxn stoichiometry) to get mol CO2 produced (theoretical); convert to g CO2.
(3.85 mol propane) (3 mol CO2/mol propane) = 11.55 mol CO2
(11.6 mol) (44.01 g/mol) = 508 g CO2 (508-511 accepted)