Empirical Formula

= based on dataUsually % composition data

Why is it important to be able to calculate the empirical formula?

• Penicillin – Antibiotic– Discovered about 80 years ago– Paved the way for other antibiotics to be created

which increased life expectancy

Why is it important to calculate the empirical formula?

• How was penicillin discovered?– By accident!– In 1928, Alexander Fleming was working at the

University of London with a common bacterium that causes boils and other infections such as blood poisoning.

– The bacteria being grown was contaminated by a bluish-green mold.

Penicllin CryosogenumThis photomicrograph shows the rod and pencil-shaped branches. The name comes from the Latin penicillus which means paintbrush.

Isolating pure Penicillin

• 1940: 2 chemists at Oxford University (Howard Florey and Ernst Chain) were able to isolate it.

• Combustion Analysis

Combustion Analysis

• Determines what elements are present and in what quantities.– What is a combustion reaction?

– Unknown + O2 H2O and CO2

– How might this method work?• Discuss with your group.

Combustion Analysis

• Law of Conservation of Mass – (Return of the Lego Atoms)– Matter cannot be created or destroyed– Whatever atoms are present in the reactants must

also be present in the products but may be combined in different forms.

– Tells us the % composition of the compound.

% Composition

• What is % composition of a compound?

• The percent of each element found in the compound.– By mass

% Composition: Penicillin

• 53.9 % Carbon• 4.8 % Hydrogen• 7.9 % Nitrogen• 9.0 % Sulfur• 6.5 % Sodium• 17.9 % Oxygen

C54%

H5%

N8%

S9%

Na7%

O18%

Sales

Determining Empirical FormulaStep 1: % to mass

• Convert mass percentages to mass.– Assume total = 100 g (easy math!)

• What are the masses for each element from the percent of each?

Step 2: Mass Moles

• How can we convert the mass of each element present to moles of each element present?

• We have grams of each element…• Convert grams moles

• How?• Divide the amount of grams by the amount of

grams/mole.

How many moles of each do you have?

• C = 4.49 mol C• H = 4.8 mol H• N = 0.56 mol N• S = 0.28 mol S• Na = 0.28 mol Na• O = 1.12 mol O

Step 3: Divide by Small• Divide the number of moles of each by the chemical with

the smallest number of moles.• C = 4.49 mol C• H = 4.8 mol H• N = 0.56 mol N• S = 0.28 mol S• Na = 0.28 mol Na• O = 1.12 mol O

– Which is the smallest number of moles?• 0.28 mol

– Divide all the mole values by 0.28

Step 3: Divide by Small• C = 4.49 / 0.28 =

– 16• H = 4.8 / 0.28 =

– 17• N = 0.56 /0.28 =

– 2• S = 0.28 / 0.28 =

– 1• Na = 0.28 /0.28 =

– 1 • O = 1.12 /0.28 =

– 4

The whole numbers are the subscripts (number of moles of each in compound)

• C = 16• H = 17• N = 2• Na = 1• O = 4• S = 1• C16H17N2NaO4S = the empirical (lowest whole

number ratio) formula for penicillin!

Empirical Formula of a Hydrate Lab

Jan 6, 2014Chemistry

Title: Empirical Formula of a Hydrate

• The following should be in your lab NB:– Purpose: • Find the empirical formula of copper(II) sulfate hydrate

by decomposition to its anhydrous form.– Materials: • hydrate, burner, balance, small test tube, ring stand,

test tube holder, scoop, test tube clamp– you might want to leave out the part about test tube and test

tube clamp until you get to the lab.

Anhydrate Hydrate

Definition: A hydrate which has had the water removed from it.(dehyrate)

A compound that has a specific number of water molecules bound to each formula unit

Applications: Desiccants hydration

Uses in everyday life:(these are examples or places these compounds are found in)

Shipping/packaging

Sodium oxide (lye with water removed) critical component of making glass.

Double pane windows (so condensation doesn’t form between panes)

Gypsum

Plaster of Paris

Epsom salt

Borax – cleaning agent (laundry)

Desalination –removing salt from water

lotions, shampoos, lip balms

Types of Hydrates

• Efflorescent Hydrates: – Spontaneously lose water

• Hygroscopic Hydrates: – Spontaneously gain water– Don’t become completely dissolved to form

solutions• Deliquescent Compounds:– absorb water from atmosphere until dissolved as a

solution

Hygroscopic Compound Use

• Silica gel packets– Often packaged with electronics and leather

goods.– Hygroscopic hydrate – removes moisture from the

air

Equation• We’re using Copper(II) sulfate

– What’s the formula for Copper(II) sulfate?• CuSO4

• And we’re going to add water…– How many waters are attached?

• We don’t know so we put nH2O– For numbers of waters, you use the prefixes we learned in naming molecular

compounds.

• The notation for hydrates uses a dot:– CuSO4 nH20

• We’re going to dehydrate it in lab, now suggest an equation:– CuSO4 nH20 CuSO4 + nH2O

Procedure

• In the lab, there is a container of Copper(II) sulfate– Take just one scoop with the scoopula and place it

into either the test tube or evaporating dish at your lab station.

• You will heat it (we’ll talk about heating) to drive off all the water – From both the Copper(II) sulfate AND the vessel

it’s in.

How do you determine n?

• What could you measure?– mass

• What is empirical formula?– Mole ratio

• It’s a mass to moles conversion• You should get an integer (that’s why we used n)…

the better your lab technique, the better the results.

• You have to figure out what mass to take in the lab.

Lab Safety• Personal Protective Equipment (PPE) Required:

– Goggles– Lab Apron

• If any Copper(II) sulfate gets on your skin, tell me and wash it off. You’ll survive

• When working with open flames/heat sources, assume EVERYTHING is hot!– Don’t touch with your hands/fingers.

• Never leave your burner unattended!• Keep anything flammable well away from the source of the fire.

• Tie back long hair• Roll up sleeves

– Watch out for drapey/flowy sleeves

Lab Techniques

Test Tube• Inspect for cracks BEFORE

heating• Only use proper test tube clamps• Point opening of test tube AWAY

from anyone’s face• Heat gently and uniformly

– Don’t hold it in just one spot over the flame.

• As condensation appears, you will move it up the tube until it is all removed.

Evaporating dish• Only use proper evaporating

dish tongs• Don’t put your face over the

dish (you don’t want to breathe in any fumes) – Remember from your lab

safety training that we treat all chemicals in the lab as hazardous!

• How to use the butane torch:– Safety off– No golden fire – incomplete combustion– Two parts to the flame

• Blue cone• Inner blue cone

– Hottest part of the flame

• Don’t mass super hot things, give them a minute to cool.• Place the test tube in the beaker to mass it.• Evaporation dish can be placed directly on analytical balance.• Use the tare (zero) button before you mass• You now have 5 min to work with your partner and come up with an outline of a

procedure you will follow.– Be sure to know why you’re doing what you’re doing and what you’re going to do

before entering the lab.