Chemical Formulas

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Chemical Formulas

TEKS 8.5 D and 8.5 F Recognize that chemical formulas are used to identify substances and determine the number of atoms of each element in chemical formulas containing subscripts. Recognize whether a chemical equation containing coefficients is balanced or not and how that relates to the law of conservation of mass. Recognize the importance of formulas and equations in representing chemical reactions– 1998 TEKS 8.9C

What do you know about chemical reactions, chemical formulas, and chemical equations? Teacher can type answers in here.

Chemical Formulas

What is a chemical formula? Chemical, or molecular, formulas are a concise way of expressing information about the atoms that constitute a particular chemical compound. Wait what? It is an expression which states the number and type of atoms present in a molecule of a substance.

Do you know what the chemical formulas are for the following substances?


Carbon dioxide

Sodium chloride or salt

Magnesium sulfate (AKA Epsom salt) MgSO 4

For something more complicated . Glucose or sugar


Definition A subscript is used to represent the number of each atom being represented. They are only used when more than one atom is being represented. If only one atom is represented, there is no subscript. In the formula for water, what is the subscript? There is only one atom of Oxygen, so it does not have a subscript. H2O H Hydrogen O Oxygen

Some exceptions exist Usually the subscript just multiplies or shows the number of atoms of a single element. If the subscript exists outside of a set of parenthesis then it will multiply the atoms of all of the elements inside the parenthesis. N(CH3) How many of each atom are there now? Answer: Nitrogen-1, Carbon-3, Hydrogen-9 N Nitrogen C Carbon H Hydrogen

Here are some molecules

In this image Hydrogen atoms are white and Nitrogen atom is red. Write the chemical formula for this molecule. Answer: NH3 This substance is ammonia

In this image the Carbon atom is black and the Oxygen atoms are red. Write the chemical formula for this molecule. Answer: CO3 This substance is carbonate

In this image blue represents Nitrogen atoms, red represents Oxygen atoms, white represents Hydrogen atoms and black represents Carbon atoms. Write the chemical formula for this molecule. Answer: C8H10N4O2 This substance is caffeine

In this image the Carbon atom is blue and the Oxygen atom is red. Write the chemical formula for this molecule. Answer: CO his substance is carbon monoxide

A harder one: In this image Nitrogen atoms are blue, the Platinum atom is grey, Chlorine atoms are green, and Hydrogen atoms are white. Write the chemical formula for this molecule. Answer: N2PtH6Cl2


Definition Coefficients appear on the left side of a chemical formula. They are used to multiply all the atoms in a compound In the following formula, which is the coefficient? Earlier we learned that the subscript 2 meant that there were two Hydrogen atoms. The coefficient 7 means there are 7 times more. How many Hydrogen atoms do we have? How many Oxygen atoms? 7H2

Chemical Equations

Chemical Equations representation of chemical reaction in equation: a representation, using chemical symbols in a form resembling a mathematical equation, of the process involved in a chemical reaction 2H2 O2 2H2O This is an example of a chemical equation. The components on the left combine together to yield (represented by the arrow) the component on the right.


Definition The quantity of a specified product obtained in a reaction or series of reactions, usually expressed as a percentage of the quantity that is theoretically obtainable


Definition What is produced. Which side represents the products? 2H2 O2 2H2O They are found on the right side of a chemical equation.


Definition The starting substances. Which side represents the reactants? 2H2 O2 2H2O They are found on the left side of a chemical equation.

Law of conservation of mass

Atoms are neither created, nor destroyed, during any chemical reaction. This means that the same number of atoms that are present after a reaction are the same number of atoms that are present before a reaction. There is only a rearrangement

Balancing chemical equations

Subscripts are never changed when balancing an equation

Step 1 Write out your “un-balanced” equation using formulas of reactants and products. CH4 O2 CO2 H2O

Step 2 Count up the atoms in the products and reactants. How many carbons, hydrogens, and oxygens are on each side? Are they equal? CH4 O2 CO2 H2O C 1 H 4 O 2 They are NOT equal C 1 H 2 O 3

Step 3 Since our carbons are ok we will not mess with those now. However, we have half the number of hydrogens in the products than we do in the reactants. What do we need to add? Where do we add it? CH44 O O22 CO CO22 H 2H CH O 2 2 C 1 C 1 O H 4 O 2 They are still NOT equal H 4 O 4

Step 4 Now we have half the number of Oxygens. What do we need to add? Where do we need to add it? Is everything equal now? CH CO 2H CH44 2O O2 CO 2H O 2 2 2 22 C 1 O C 1 H They ARE now balanced H 4 O 4

Practice Time

2H H22 O22 H 2H O O 2 2

Fe Cl2 FeCl 2Fe 3Cl 2FeC 3 2 l3

Cu 2AgNO33 Cu(NO Cu(NO33))2 2 Cu AgNO 2 Ag Ag

Zn 2HCl ZnCl Zn HCl ZnCl22 H H22

Pb(NO3)22 2AlCl AlCl3 3PbCl PbCl22 Al(N 3Pb(NO 2Al(N O O33))33

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