Chemical Calculations: Formula Masses, Moles, and Chemical Equations

Formula Mass

Sum of all of the atomic masses in the formula of a substance

Use Atomic Mass Units (AMU) 1 AMU = mass of 12C / 12

Where do you find the atomic masses?(on the Periodic Table)

(Sometimes referred to as formula weight or molecular weight)

Determine the Formula Mass

Remember – atomic masses are based upon measurement

H2O

H2SO4

(NH2)2CO

The Mole

The Mole

The unit of “amount” in chemistry

6.02 x 1023 of anything (Avogadro’s Number)

The number of Carbon atoms in 12 gram of 12C

Mole Calculations

6.02 x 1023 objects 1 mole

1 mole 6.02 x 1023 objects

How many molecules are in 4.7 moles of aspirin?

If you have 7.248 x 1024 atoms of carbon, how many moles of carbon atoms do you have?

If you have 17.4 moles of H20, how many moles of hydrogen do you have?

Molar Mass

The mass of one mole of a substance

Same as formula mass except change AMU to g/mole

Determine the molar mass of:

H2O

NH3

Ca3(PO4)2

Molar mass is a CONVERSION FACTOR

Allows conversion from moles to grams and from grams to moles.

Plus…. it allows you to weigh out grams rather than count out moles.

Molar Mass Conversions

How many moles of water do you have if you have 32.062 grams of water?

If you need 2.0 moles of NH3 for an experiment, how many grams do you need to weigh out?

Chemical Equations

A way of accurately communicating what occurs during a chemical process

Identifies the reactants (starting substances)

Identifies the products (final substances)

Takes into account the fact that mass is conserved and atoms are not created or destroyed.

Combustion of Hydrogen

H2 + O2 H2O

Does this accurately identify the reactants and product?

Is there conservation of matter? (same number of each type of atom on both sides)

Balancing equations

Adding numbers to one or both sides of an equation to ensure equal number of each type of atom?

What numbers can we change?

Subscripts?

Coefficients?

Method for balancing an equation

1) Make sure reactants and products are correct

2) Examine to find out which elements are not balanced. Pick one to balance by changing coefficients.

Try to pick one that is only found in one molecule on each side of the equation.

3) Pick a second element. Sometimes it helps to pick one that already has a new coefficient on the other side.

4) Continue the process until the equation is balanced

5) Check your result by adding up all atoms on both sides of the equation

Hints: Sometimes lone elements can be used last of all to “clean up with”Sometimes you can see multiples that can be used

X2 Y3

A “3” works here A “2” works here

More hints:

Sometimes you can use fractions to help

3/2 X2 X3

But… the final balanced equation must have whole number coefficients in their lowest whole number ratios

3 X2 2 X3

What do the coefficients of a balanced equation represent?

2 H2 + O2 2 H2O

Grams of each substance? No

Molecules of each substance? Yes

Moles of each substance? Yes

Stoichiometry Converting amounts of one substance into amounts

of another substance in a balanced chemical equation.

Mole/Mole ratioUses equation coefficients and allows you to convert moles of one substance into moles of any other substance in the equation

Molar Mass

Allows you to convert grams of a substance into moles of a substance and vice versa.

Simplest stoichiometric conversions

Moles of A into Moles of B Uses mole ratio (coefficients of balanced

equation)

Next simplest conversion

Grams of A to Moles of B Uses molar mass of A and then mole ratio of

A to B

Finally… converting Grams of A to Grams of B

Grams of A to Moles of A (molar mass of A) Moles of A to Moles of B (mole ratio) Moles of B to Grams of B (molar mass of B)

Lets try a bunch of these….Lets try a bunch of these….

Make sure you have a balanced equation

Use the molar mass of a substance to convert moles of it to grams of it and vice versa.

Use the mole ratio (coefficients) to convert moles of one substance to moles of another substance