The Alkali Metals

The first group on the left of the Periodic Table, Group 1 (IA) is known as the Alkali Metals. The alkali metals are highly reactive metals that do not exist by themselves in nature. They are highly reactive because they have just one valence electron, which they can easily give up to form an ionic…

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Structure of the Modern Periodic Table

The Modern Periodic Table is arranged in order of increasing atomic number, which is the number of protons in the nucleus of an atom. The lightest atom is Hydrogen, in the upper left-hand corner, with an atomic number of 1. This means that hydrogen contains only 1 proton in its nucleus. The heaviest naturally occurring…

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History of the Periodic Table

The Greeks were the first to float the idea that matter was composed of elements. Aristotle believed these elements to be Earth, Fire, Air and Water, whilst Democritus theorized an atomic theory of matter — that all matter was composed of indivisible units or “atomos.” (”Atomos” means indivisible). Antonie Lavoisier in the 1700s was the…

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What Did Rutherford’s Gold Foil Experiment Show about Atomic Structure?

What Did Rutherford’s Gold Foil Experiment Show about Atomic Structure? Ernest Rutherford and his team, Geiger and Marsden, aimed a beam of alpha particles from a radioactive uranium source at a piece of gold foil, in front of a screen. The screen would flash a green light whenever it was hit by one of these…

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What did J. J. Thomson Discover?

JJ Thomson, full name Sir Joseph John Thomson, is credited with the discovery of the electron, which he called a corpuscle.  His experiment involved a cathode ray tube (CRT) which is a high-vacuum tube consisting of a beam of electrons which could be deflected using either an electric field or magnetic field.    Because the beam…

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Stoichiometry Examples

Here are some practice stoichiometry problems.  They include such conversions as grams-to-liters gas at STP, molecules-to-grams, grams-to-kJ (kilojoules) and kJ-to-molecules, besides the usual moles-to-moles, grams-to-moles, moles-to-grams and grams-to-grams stoichiometry examples. The key to solving any and all stoichiometry problems is to convert everything to moles. If you’re given grams, turn it into moles. If you’re…

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Stoichiometry Help

Sample Problem N2 + 3H2 –> 2NH3 How many moles of ammonia can be produced from 7.00 moles of hydrogen? We want to find the number of moles of ammonia that can be produced from 7.00 moles of hydrogen. WANTED: ? moles NH3 GIVEN:  7.00 moles H2 This is a moles to moles problem, as we’re…

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How Much Does a Mole Weigh?

A mole of carbon, or 6.02 x 1023 atoms of carbon, weighs 12 grams.  But how much a mole weighs depends on what it’s made of.  Moles of different atoms weigh differently, because different atoms weigh differently.  We refer to the mass of 1 mole of an atom or molecule as its molar mass.  So…

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What is a “mole?”

What is a “mole?” The “mole” is an SI (System International) unit of measurement based on carbon-12.  It is also referred to as Avogadro’s number.  A mole is the number of atoms in 12 g of carbon-12.  And that’s a really large number.   The number of atoms in 12 g of carbon-12 is 6.02 ×…

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Converting Molecules to Moles

In many stoichiometry problems, you will be given a certain number of molecules (or atoms, ions, formula units), and asked to convert that number into moles. To do this, you need to realize that 1 mole is equal to 6.02 x 1023 particles, regardless of the type of particle. So, if I have 6.02 x 1023…

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How to Determine Average Atomic Mass

Average atomic mass, the mass indicated on the periodic table, is an average of the masses of all the naturally occurring isotopes, weighted by their abundance.  These are the naturally occurring isotopes of carbon, and their abundance in nature. Isotope Percent Abundance Atomic Mass 12C 98.9% 12.000000 13C 1.1% 13.003355 14C* 0 14.003241   To…

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Average Atomic Mass

1 mole of Carbon-12 has a mass of 12.00 grams.  The Periodic Table, however, lists carbon’s atomic mass as 12.011 grams per mole.  The reason is that there are naturally occurring isotopes of carbon that have masses other than 12.00 grams.  The atomic mass listed on the periodic table is the average of all these…

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How Did They Come Up with Avogadro’s Number?

Avogadro’s number was named to honor the Italian chemist Amadeo Avogadro, but was certainly not discovered by him.  He was honored with such a moniker because of his work in identifying that equal volumes of gases at the same temperature and pressure contained equal numbers of particles. Avogadro’s Hypothesis, that equal volumes of gases contain…

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How to Write Net Ionic Equations

A net ionic equation is a chemical equation that shows only the reacting species in a solution.  All other species that are the same on each side of the equation are called spectator ions, and are omitted from the final equation.  Generally speaking, you can identify the reacting species by determining which compound will form…

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Redox Titration

Titration Isn’t Just for Acids and Bases Potassium permanganate, KMnO4, is often used as a titrant in a redox titration because it’s an excellent oxidizing agent, due mostly to its multiple oxidation states. Iron (II) salts work especially well with KMnO4 because the endpoint is crystal clear – the analyte turns from clear to pink/purple.…

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AP Chemistry Quantitative Analysis Using Titration

The AP Chemistry exam expects you to know how to solve quantitative analysis problems using the two laboratory techniques of gravimetric analysis and titration.  In this post, I will teach you how to solve titration problems to determine the amount of analyte present. In titration, an unknown concentration of analyte is determined by measuring the…

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AP Chemistry Determining the Identity of an Unknown Metal Carbonate using Gravimetric Analysis

In one of the classic AP Chemistry labs, determining the identity of an unknown metal carbonate using gravimetric analysis, the Group IIA metal carbonate, M2CO3 (where M represents the unknown Group IIA metal) was precipitated out of solution using CaCl2. Let’s say the initial mass of the metal carbonate was 1.850 grams, and that the…

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AP Chemistry 2008 Free Response Question 2 Form A

Question #2 in the 2008 AP Chemistry Exam Version A has a gravimetric analysis problem.  It goes as follows: A student is given 2.94 g of a mixture containing anhydrous MgCl2 and KNO3 . To determine the percentage by mass of MgCl2 in the mixture, the student uses excess AgNO3(aq) to precipitate the chloride ion…

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An Involved Gravimetric Analysis Problem

Last post I left you off with a gravimetric analysis problem to solve.  The problem was as follows: A 0.8870 g sample containing only NaCl and KCl was treated with AgNO3. The AgCl formed had a mass of 1.913 g. Calculate the %Na and %K in the sample. Let’s first write up the chemical equation:…

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Using Quantitative Analysis in AP Chemistry

There are two distinct methods to determine the amount of substance in a sample that the AP Chemistry exam expects you to know.  They are: Gravimetric Analysis Titration In gravimetric analysis, an unknown concentration of analyte is determined by either (a)    precipitating it out of solution (b)   depositing it as a pure metal using electrolysis,…

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Taking Molecule “Selfies”

The 2014 Nobel Prize in Chemistry was awarded to Eric Betzig of Howard Hughes Medical Institute, US, Stefan Hell of the Max Planck Institute for Biophysical Chemistry, Germany, and WE Moerner of Stanford University, US, for their work in imaging particles as small as molecules. Previously, no pictures could be taken of molecules because of…

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How to Select the Appropriate Type of Spectroscopy for AP Chemistry

One of the new learning objectives for the new AP Chemistry exam (as of 2013) is learning objective 1:15: The student can justify the selection of a particular type of spectroscopy to measure properties associated with vibrational or electronic motions of molecules. (See SP 4.1; Essential Knowledge 1.D.3) So how exactly does a student do…

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Interpreting Photoelectron Spectroscopy (PES)

Last post I left you with the following Photoelectron Spectroscopy (PES) spectrum for you to interpret.  As a quick review, recall that in PES electrons are bombarded with enough high energy light to eject certain electrons from the atom.  The relative number of electrons ejected per energy required to eject them – their ionization energy…

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Photoelectron Spectroscopy on the New AP Chemistry Exam

Photoelectron Spectroscopy on the new AP Chemistry Exam Photoelectron Spectroscopy will also be tested on the new AP Chemistry test.  Photoelectron Spectroscopy (PES) is used to measure the energy and number of electrons in an atom.  Electrons are bombarded with high energy light such as x-ray or ultraviolet, enough to eject some electrons from the…

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Mass Spectrometry Continued

Last time I left you with the following problem: Determine the average atomic mass and identity of the sample from the mass spectrometer reading below. In the above sample, I’m going to make liberal approximations with the readings. I can see that 50% of the sample has an atomic mass of 90, approximately 10% has…

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Mass Spectrometry in the new AP Chemistry Exam

A mass spectrometer produces spectra of the masses of the atoms or molecules in a sample.  On the new AP Chemistry exam, students will be presented with mass spectrometry readings and expected to interpret them to determine the average atomic mass of a pure substance. When a pure substance is introduced into the mass spectrometer…

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AP Chemistry New Curriculum Changes

Those taking the AP Chemistry exam in 2014 and beyond will need to know about the following additions to the AP Chemistry curriculum.  These new topics are: Photoelectron spectroscopy (PES) Mass spectroscopy Coulomb’s Law and its effect on all types of energetic interactions, including periodic trends Particulate representations, especially of strong and weak acids and…

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Physics WS 4.5 KEY

WS 4.5 #1-3 KEY WS 4.5 #4-5 KEY WS 4.5 #7-14 KEY

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Do-It-Yourself Flame Tests

Flame tests are one of the standard laboratories in high school chemistry.  In this video, I show you how to do flame tests yourself, at home, using a few simple household ingredients.  These ingredients are: Flame source (I use an alcohol burner) Paper clip attached to a cork (to serve as the nichrome wire) Denatured…

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Science Education and the Common Core

Here we go again.  Welcome the new hero of education — Common Core.  This school year, 2013-2014, 45 out of 50 states will adopt the Common Core standards and the financial rewards that come with it.  Common Core is heralded as the lone savior that stands between the United States and utter economic collapse.  So…

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