February 09, 2015

Determining Chemical Formulas An EMPIRICAL formula consists of the symbols for the elements combined in a compound with subscripts showing the smallest whole-number mole ratio of the different atoms in the compound.

Calculation of Empirical Formulas Calculating empirical from a molecular formula is easy. What you need to do is find the greatest common factor and divide every subscript by that GCF.

For example: tetraphosphorous pentoxide - P4O10 trichlorisocyanuric acid - O3C3N3Cl3 B4H12

February 09, 2015

Calculation of Molecular Formulas The molecular formula is the actual formula of a molecular compound. Remember, an empirical formula may or may not be a correct molecular formula.

For example: The empirical formula of diborane is BH3 Any multiple of BH3 - B2H6, B3H9, B4H12 and so on represents the same RATIO of B atoms to H atoms. The ratio is 1B:3H In this example C2H4 this is called ethene and C3H6 is called cyclopropane. They are VERY DIFFERENT COMPOUNDS yet, they have the same identical empirical formula! The ratio of Carbon to Hydrogen in both of these compounds is 1C: 2H

It’s time to make pancakes! The recipe calls for 1 egg, 1 cup of mix, 1 cup of milk, 2 tablespoons of oil, 1 1⁄2 tablespoons of vanilla extract and 1/3 teaspoon of nutmeg. This will make sixteen 4” diameter pancakes. This will serve four people quite nicely, and make for a delicious breakfast. But wait! Your grandparents and an aunt and uncle are coming to visit! You now have 8 people coming to breakfast, and the recipe only gives you instructions for four servings!!! Whatever shall you do? Easy! Use RATIOS to double the recipe and make enough to satisfy everyone’s morning hunger!

Notice how the proportions of the ingredients AND the pancakes are preserved?

February 09, 2015 The mole? Mole, abbreviated mol is the amount of a substance that contains as many particles as there are atoms in exactly 12 grams of carbon-12. Avogadro's number is 6.02x1023 This means that EXACTLY 12g of carbon-12 contains 6.02 x 1023 atoms = 1 mole To give you an idea on how large Avogadro's number is, If every living person on Earth (6 billion or so) worked to count the atoms on 1 mole of an element, and if each person counted 1 atom per second, it would take 3 million years for all the atoms to be counted. This is all called STOICHIOMETRY - comes fro the Greek "stoicheion" meaning element and "metron" meaning measure. It really all boils down to careful analysis and relationships of substances.

February 09, 2015 CONVERTING MOLES TO GRAMS AND GRAMS TO MOLES 1) Determining how many moles of substance there are in a certain mass of a substance To convert grams to moles, divide by the formula mass: (given g) / (g / mole) = mole How many moles of KCl are there in 182.9 grams of KCl? 1) Find the formula mass of KCl: 2) Convert moles = (grams/ formula mass)

How many moles of Ca(NO3)2 are there in 45.5 grams of Ca(NO3)2? 1) Find the formula mass of Ca(NO3)2:

2) Convert moles = (grams/ formula mass)

2) Determining the mass of a certain number of moles of substance. To convert moles to grams, multiply by formula mass: (given moles X g/ moles = g)

A 2.45 mole sample of KCl will have what mass in grams?

February 09, 2015 Mole Ratios

This is also true of chemical reactions.

The coefficients in front of each species lets you know what the proportional number of moles of reactant needed is to make a proportional number of moles of product.

N2 + 3H2 ➼ 2NH3 What this means is that if nitrogen and hydrogen are reacted in a 1:3 mole ratio, the amount of ammonia you will produce from the reaction is 2 moles. This is important to know, because if your company gets a call for 10 moles of ammonia, you can use that 1:3 2 proportion to determine how many moles of nitrogen and hydrogen you have to react together in order to make the 10 moles of ammonia. Let’s see how this scales up:

N2 + 3H2 ➼ 2NH3

For the reaction N2 (g) + 3H2 (g) 2NH3 (g), how many moles of NH3 will be formed if 6.0 moles of N2 are completely reacted with H2?

For the reaction 2Na + 2H2O → 2NaOH + H2, how many moles of Na are needed to make 4.0 moles of H2?

For the reaction 4Al + 3O2 → 2Al2O3, how many moles of Al2O3 will form if 6.0 moles of Al are completely reacted with O2?

2 NaCl (aq)+ Pb(NO3)2 (aq) → 2 NaNO3 (aq) + PbCl2 (s), how many moles of PbCl2 precipitate will form when 5.0 moles of Pb(NO3)2 are completely reacted with NaCl?

Mole Ratios are EXACT ! It is a conversion factor that relates the amounts in moles of any 2 substances involved in a chemical reaction. The information is obtained DIRECTLY from the balanced chemical equation.

In a spaceship, the carbon dioxide exhaled by astronauts can be removed by its reaction with lithium hydroxide (LiOH), according to the following chemical equation. CO2(g) + 2LiOH(s) ➤ Li2CO3(s) + H2O(l) How many moles of lithium hydroxide are required to react with 20 mol CO2, the average amount exhaled by a person each day?

Ammonia, NH3 is widely used as a fertilizer and in many household cleaners. How many moles of ammonia are produced when 6 mol of hydrogen gas react with an excess of nitrogen gas?

The decomposition of potassium chlorate, KClO3, is used as a source of oxygen in the laboratory. How many moles of potassium chlorate are needed to produce 15 mol of oxygen gas?

February 09, 2015

To determine the molecular formula of a compound, you must know the compound's formula mass