How do chemists determine the formula of compounds?

What is the formula of iron oxide? Remember the steel wool lab? How would you experimentally determine the formula? Do you know the mass of iron used? (g Fe) Do you know the mass of oxygen used? (g O) Could you convert both grams to moles? Could you find the molar ratio of Fe to O? What would that tell you about the formula?

Chemical Formulas of Compounds What do formulas tell us?What do formulas tell us? NO 2 1 atom of N for every 2 atoms of O NO 2 1 atom of N for every 2 atoms of O 5 atoms of N for every 10 atoms of O5 atoms of N for every 10 atoms of O 100 atoms of N for every 200 atoms of O100 atoms of N for every 200 atoms of O 6.022x10 23 atoms of N for every x10 23 atoms of O6.022x10 23 atoms of N for every x10 23 atoms of O This is the same as:This is the same as: 1 mole of N : 2 moles of O atoms1 mole of N : 2 moles of O atoms Formulas give the relative numbers of atoms or moles of each element in a formula unit - always a whole number ratio (the law of definite proportions).Formulas give the relative numbers of atoms or moles of each element in a formula unit - always a whole number ratio (the law of definite proportions). If we know or can determine the relative number of moles of each element in a compound, we can determine a formula for the compound.If we know or can determine the relative number of moles of each element in a compound, we can determine a formula for the compound.

Types of Formulas Empirical FormulaEmpirical Formula The formula of a compound that expresses the smallest whole number ratio of the atoms present. OR the LOWEST WHOLE NUMBER RATIO OF MOLES –Ionic formula are always empirical formula –Molecular Formula The formula that states the actual number of each kind of atom found in one molecule of the compound. We will come to this later

What if I had Manganese? What are the possible formulas for manganese chloride? Which one will you make if you add chloride ions to Mn metal?

To obtain an Empirical Formula 1.Determine the mass in grams of each element present, if necessary. 2.Calculate the number of moles of each element. 3.Divide each by the smallest number of moles to obtain the simplest whole number ratio. 4.If whole numbers are not obtained * in step 3), multiply through by the smallest number that will give all whole numbers * Be careful! Do not round off numbers prematurely

A sample of a brown gas, a major air pollutant, is found to contain 2.34 g N and 5.34g O. Determine a formula for this substance. require mole ratios so convert grams to moles 2.34g of N x 1 mol N = moles of N 2.34g of N x 1 mol N = moles of N g N g N 5.34 g of O x 1 mol O = moles of O g of O g of O Formula: Formula:

Calculation of the Molecular Formula A compound has an empirical formula of NO 2. The colourless liquid, used in rocket engines has a molar mass of 92.0 g/mole. What is the molecular formula of this substance?

Example A sample of urea is decomposed into its elements. The following products are measured: nitrogen: g hydrogen:.161 g carbon:.480 g oxygen:.640 g

Steps in determining the formula of urea 1.Find the number of moles of each element present: nitrogen: g x ( )= hydrogen: g X ( )= carbon: g X ( )= oxygen: g X ( )=

2. Find the lowest whole number ratio in moles (the empirical formula)

The subscripts in the formula will reflect the smallest whole number ratio between the moles of each of the elements present.

Empirical Formula from % Composition A substance has the following composition by mass: % Na ; % B ; % H What is the empirical formula of the substance? Consider a sample size of 100 grams This will contain grams of B and grams H Determine the number of moles of each Determine the simplest whole number ratio

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Another example When 2.30 g of steel wool (mostly iron) is burned, a white ash is produced with a mass of 3.29 g. Assuming that the gray ash is an oxide of iron, what is the formula of this new substance?

1. Determine the number of moles of iron and oxygen are present

2. Find the smallest whole number ratio of the moles of iron and oxygen

3. Write a formula which reflects the smallest whole number ratio between the atoms of iron and oxygen.

What is the percent carbon in C 5 H 8 NO 4 (the glutamic acid used to make MSG monosodium glutamate), a compound used to flavor foods and tenderize meats? a) 8.22 %C b) 24.3 %C c) 41.1 %C Percent Composition