Isotopes
Definition
Quantitative Analysis
Mass Spectrometer
Number of the periodic table is determined by…
Formula
Question 1: It has been found by mass spectrometric analysis that in nature the relative
abundances of various isotopes of silicon are 92.23% 28Si, 4.67% 29Si, 3.10% 30Si. If
the masses are 28Si = 27.97693, 29Si = 28.97649, 30Si = 29.97377, calculate the atomic
mass of silicon.
Question 2: Naturally occurring carbon consists of two major isotopes, 12C and 13C.
What are the percent abundances of the two isotopes in a sample of carbon whose atomic
mass is 12.01112?
Question 3: 35-Cl and 37-Cl are the only naturally occurring chlorine isotopes. What
percent distribution accounts for the atomic mass? Isotopic masses: 35 Cl = 34.96885
and 37 Cl = 36.96590
Average Atomic mass of chlorine – 35.4527
Moles
Average atomic mass is how much one mole of that element weighs. Numbers
determined by relative mass. Standard for the periodic table:
What if you don’t have a pure element, but a compound like water?
Try for the following compounds:
NaHCO3
NaCl
AgCl
Lithium Hydroxide
AlF3
KC2H3O2
SO3
Co(NO3)3
Phosphorous pentachloride
CO
xenon hexafluoride
Percent Composition
1.) A mixture of compounds includes the compound Co(AlO2)2. What is the percentage
composition of the elements in this compound?
2.) The compound mannitol, C6H8(OH)6, is used as a sweetener in some dietetic foods.
What is the percentage composition of the elements in this compound?
Empirical Formulas
6.) A laboratory technician ran an analysis of a germicide stocked by a janitorial service.
The technician determined that the effective ingredient in the germicide was a compound
that was 33% sodium, 36% arsenic, and 31% oxygen. What is the formula for this
compound?
Molecular Formulas
From the following empirical formulas, determine the molecular formula of the
compound:
empirical formula
molecular formula mass
SNH
(188.35 g/mol)
NPCl2
(347.64 g/mol)
CoC4O4
(341.94 g/mol)
SN
(184.32 g/mol)
A white powder is analyzed and found to contain 43.64% phosphorous and 56.36%
oxygen by mass. The compound has a molar mass of 283.88 g/mol. What are the
compounds empirical and molecular formulas?
Caffeine, a stimulant found in coffee, tea, and chocolate, contains 49.48% carbon, 5.15%
hydrogen, 28.87% nitrogen, and 16.49% oxygen by mass and has a molar mass of 194.2
g/mol. Determine the molecular formula of caffeine.
Chemical Reactions and Stoichiometry
Ba + O2 -->
C + S
BaO
--> CS2
KClO3 -->
Zn + HCl
KCl + O2
-->
ZnCl2 + H2
You mix 5.0 grams of Silver nitrate with excess sodium chloride. A white solid called
Silver chloride appears. How much silver chloride was created?
A 15 g iron nail is left outside to rust. How much will the rusted nail weigh after all the
iron is allowed to react with the air?
You have a 3.5 g sample of copper that is placed in a container of nitric acid. How much
hydrogen is liberated from this reaction?
If you mix a 5.0 g piece of Aluminum together with
3.5 g of CuCl2, how much pure copper can you reclaim?
Which reactant will you have remaining?
Limiting and Excess Reactants
2.5 grams of potassium iodide and 3.5 g lead nitrate are mixed together in water and
allowed to react. How much of the yellow lead iodide is created?
Stoichiometry and Empirical Formulas
Combine both processes to come up with a final answer.
Vanillin, a flavoring agent, is made up of carbon, hydrogen, and oxygen atoms. When a
sample of vanillin weighing 2.500 g burns in oxygen, 5.79 g of carbon dioxide and 1.18 g
of water are obtained. What is the empirical formula of vanillin?
Methyl salicylate is a common “active ingredient” in liniments such as Ben-Gay. In is
also known as oil of wintergreen. It is made up of carbon, hydrogen, and oxygen atoms.
When a sample of methyl salicylate weighing 5.287 g is burned in excess oxygen, 12.24
g of carbon dioxide and 2.522 g of water are formed. Find the empirical formula for oil
of wintergreen.